user2277: In Theravada tradition, after one becomes a monk. Does he still call his parents and relatives the same as before he ordains. Or does he call them like everybody else? Does he call his parents "mom" and "dad", his siblings "brother" and "sister",...?

user-1: No, householder, such is not proper for one having left home.
He would address them proper like he would address other laypeople. Householder, good man, Brahman... Here in Cambodia the usual addressing of householders by monks is Nyom (Ñom(a)), as a polite and proper third person approach when speaking direct. For mother and father, Nyom would be attached with male and female and understanding would come from attentive following the context. Speaking about third people, Upasaka, Upasika or Nyom attached with name, male or female, would be used.
Speaking general about lay people "householders" would be used. Often the use of Buddha-Parisatas (gathering around the Buddha) would be used to speak about lay or mixed groups.
It's a special hard issue for modern and wester people, even more hard to bear for those strongly marxistic influenced and/or be-favored by monks seeking after followers, something very wrong and trading introduced to non-traditional societies. Pretty annoying for devoted followers who have joy to keep their objects reminding on their refuge in Sublime position.
One might be more interested in matters of respect, fundamental important for a real refuge and to grow toward such: Respect and veneration, apacāyana

Kandarayana Sutta
..."Brahman, the Blessed One — the one who knows, the one who sees, worthy & rightly self-awakened — has declared the level of one who is venerable and the level of one who is a youngster. Even if one is venerable — 80, 90, 100 years old — yet if one partakes of sensuality, lives in the midst of sensuality, burns with sensual fever, is chewed up by sensual thoughts, and is eager in the search for sensuality, then one is reckoned simply as a young fool, not an elder.
"But if one is a youngster, youthful — a black-haired young person endowed with the blessings of youth in the first stage of life — yet does not partake of sensuality, does not live in the midst of sensuality, does not burn with sensual fever, is not chewed up by sensual thoughts, and is not eager in the search for sensuality, then one is reckoned as a wise elder."
When this was said, Kandarayana the brahman rose up from his seat, arranged his cloak over one shoulder, and bowed down at the feet of the monks who were youngsters, [saying,] "You, sirs, are the venerable ones, standing on the level of those who are venerable. We are the youngsters, standing on the level of those who are youngsters.

How ever, not to be mistaken! Wise praise the gratitude and support toward parents and in this manner the Buddha gave many allowances to uphold this. Near relatives are also the only lay people which a Bhikkhu would be allowed to address in need, if wishing. He may also leave even Vassa (rain retreat) for seven days for his patents and siblings. And he would be allowed to share what is given to him in faith to support parants in need.
Since a general gard to bear topic, that of not equal or even contrary usuals, one should not hesitage to ask further, best here, on this matter, most are scared to take on it, living often in improper dependency of lay people communities, fearing worldly and support disadvantages.
(Note that this is not given for trade, exchange, stacks, entertainment and akusala deeds, but as a share of merits and to continue such for release)

user18: It may depend on the tradition, but in the Pali suttas, the language changes unequivocally to more formal modes of address like "gahapati" (householder), "upaasaka/upaasikaa" (lay follower), etc.

Then the clansman Raṭṭhapāla rose from his seat, and after paying homage to the Blessed One, he departed, keeping him on his right. He went to his parents and told them: “Mother and father, as I understand the Dhamma taught by the Blessed One, it is not easy while living in a home to lead the holy life, utterly perfect and pure as a polished shell. I wish to shave off my hair and beard, put on the yellow robe, and go forth from the home life into homelessness. Give me permission to go forth from the home life into homelessness.”

...

Then the venerable Raṭṭhapāla’s former wives clasped his feet and said to him: “What are they like, my lord’s son, the nymphs for whose sake you lead the holy life?”
“We do not lead the holy life for the sake of nymphs, sisters.”
“Our lord’s son Raṭṭhapāla calls us ‘sisters,’” they cried and right there they fainted.
Then the venerable Raṭṭhapāla told his father: “Householder, if there is a meal to be given, then give it. Do not harass us.”

-- (MN 82 - Bodhi, trans)

2277: <p>In Theravada tradition, after one becomes a monk. Does he still call his parents and relatives the same as before he ordains. Or does he call them like everybody else? Does he call his parents "mom" and "dad", his siblings "brother" and "sister",...?</p>
 -1: <p>No, householder, such is not proper for one having left home.</p>

<p>He would address them proper like he would address other laypeople. Householder, good man, Brahman... Here in Cambodia the usual addressing of householders by monks is Nyom (Ñom(a)), as a polite and proper third person approach when speaking direct. For mother and father, Nyom would be attached with male and female and understanding would come from attentive following the context. Speaking about third people, Upasaka, Upasika or Nyom attached with name, male or female, would be used.</p>

<p>Speaking general about lay people "householders" would be used. Often the use of Buddha-Parisatas (gathering around the Buddha) would be used to speak about lay or mixed groups.</p>

<p>It's a special hard issue for modern and wester people, even more hard to bear for those strongly marxistic influenced and/or be-favored by monks seeking after followers, something very wrong and trading introduced to non-traditional societies. Pretty annoying for devoted followers who have joy to keep their objects reminding on their refuge in Sublime position.</p>

<p>One might be more interested in matters of respect, fundamental important for a real refuge and to grow toward such: <a href="http://sangham.net/index.php/topic,2015.msg13980.html#post_ea3" rel="nofollow noreferrer">Respect and veneration, apacāyana</a></p>

<blockquote>
  <p><em><a href="http://zugangzureinsicht.org/html/tipitaka/an/an02/an02.038.than_en.html" rel="nofollow noreferrer">Kandarayana Sutta</a></em></p>
  
  <p>..."Brahman, the Blessed One — the one who knows, the one who sees, worthy &amp; rightly self-awakened — has declared the level of one who is venerable and the level of one who is a youngster. Even if one is venerable — 80, 90, 100 years old — yet if one partakes of sensuality, lives in the midst of sensuality, burns with sensual fever, is chewed up by sensual thoughts, and is eager in the search for sensuality, then one is reckoned simply as a young fool, not an elder.</p>
  
  <p>"But if one is a youngster, youthful — a black-haired young person endowed with the blessings of youth in the first stage of life — yet does not partake of sensuality, does not live in the midst of sensuality, does not burn with sensual fever, is not chewed up by sensual thoughts, and is not eager in the search for sensuality, then one is reckoned as a wise elder."</p>
  
  <p>When this was said, Kandarayana the brahman rose up from his seat, arranged his cloak over one shoulder, and bowed down at the feet of the monks who were youngsters, [saying,] "You, sirs, are the venerable ones, standing on the level of those who are venerable. We are the youngsters, standing on the level of those who are youngsters.</p>
</blockquote>

<p>How ever, not to be mistaken! Wise praise the gratitude and support toward parents and in this manner the Buddha gave many allowances to uphold this. Near relatives are also the only lay people which a Bhikkhu would be allowed to address in need, if wishing. He may also leave even Vassa (rain retreat) for seven days for his patents and siblings. And he would be allowed to share what is given to him in faith to support parants in need.</p>

<p>Since a general gard to bear topic, that of not equal or even contrary usuals, one should not hesitage to ask further, best <a href="http://sangham.net/index.php?action=forum" rel="nofollow noreferrer">here</a>, on this matter, most are scared to take on it, living often in improper dependency of lay people communities, fearing worldly and support disadvantages.</p>

<p><em>(Note that this is not given for trade, exchange, stacks, entertainment and akusala deeds, but as a share of merits and to continue such for release)</em></p>
 18: <p>It may depend on the tradition, but in the Pali suttas, the language changes unequivocally to more formal modes of address like &quot;gahapati&quot; (householder), &quot;upaasaka/upaasikaa&quot; (lay follower), etc.</p>
<blockquote>
<p>Then the clansman Raṭṭhapāla rose from his seat, and after paying homage to the Blessed One, he departed, keeping him on his right. He went to his parents and told them: “Mother and father, as I understand the Dhamma taught by the Blessed One, it is not easy while living in a home to lead the holy life, utterly perfect and pure as a polished shell. I wish to shave off my hair and beard, put on the yellow robe, and go forth from the home life into homelessness. Give me permission to go forth from the home life into homelessness.”</p>
</blockquote>
<p>...</p>
<blockquote>
<p>Then the venerable Raṭṭhapāla’s former wives clasped his feet and said to him: “What are they like, my lord’s son, the nymphs for whose sake you lead the holy life?”</p>
<p>“We do not lead the holy life for the sake of nymphs, sisters.”</p>
<p>“Our lord’s son Raṭṭhapāla calls us ‘sisters,’” they cried and right there they fainted.</p>
<p>Then the venerable Raṭṭhapāla told his father: “Householder, if there is a meal to be given, then give it. Do not harass us.”</p>
</blockquote>
<p>-- <em>(MN 82 - Bodhi, trans)</em></p>
