Postdoc position not specifying about required recommendation letters As I'm approaching to the end of my PhD, I recently applied for a postdoc position. Before applying, I contacted the scientist offering the position, sending my resume and some publications which are of interest for the position. He encouraged me to apply as my experience would be important for his project and he also scheduled an interview at the end of this month, saying that he's making a decision in April. 

However, he didn't ask me for recommendation letters, he just told me to apply online just to make my application official. The online form only requires a resume, any other document is optional.

Neither him nor the position advertisement specyfied any information about recommendation letters (e.g., the minimum number, or whether I should upload them online or just provide the emails of my references). 

Is this common in research (the position is in the USA)? Should I ask him more info now or wait untill the interview?
While it seems unusual, I'm going to guess that he knows what he wants and needs. I wouldn't send things that aren't asked for in this case, as you seem to have already made it over the line. An accidental statement in a letter might actually hurt you, but I don't see how you can be "more" accepted than accepted. 

If asked, send them of course. But you will likely learn more at the interview. 

Depending on his funding, he may have wide latitude in who to hire and how to do it.