Follow up email to a potential PhD supervisor I have contacted a potential PhD supervisor and he sent me back a positive email and asked "have you contacted anyone else in the department or just me at this stage?". 

I have replied to the email in a very short notice and stated that I have directly contacted him after careful consideration of the academic staff profile because I can more easily merge with the research theme he works on.

It has been 2 weeks but haven't received a reply, should I write a follow up email? If yes, what should I write in it?
Whether it is a potential PhD supervisor, committee member, or employer I think it is important at this stage to simply send a polite kind reminder. 
I have been in many of these situations and did not know how to handle it properly. I found it worked best for me to send a reminder 1 week afterwards your conversation; best timing: first thing in the morning, avoid weekends; so you catch his/her attention without pushing/forcing the person. Send two/three reminders max (one per week) if there are answers at that stage drop it.
That would be a short template of this form (short is important there)

Dear XXX,

I hope you are having a pleasant day. 
I was wondering if you had the opportunity to review my email sent out on xx.xx. I know you are particularly busy at the moment but I would be really grateful if you can please spare a few minutes of your time to read it through.
Have a great day ahead.

Best regards,

First name Name

Regarding the formality, it depends, some people like to be called Prof. or Dr., other prefer to be informal after the first contact. 
Whenever in doubt, use the polite form.
For the first sentence you can be more specific: winter, summer, vacations, back from a conference, etc.

For the salutation, it will depends on the first exchange of emails, again some people prefer to be more formal other less. You will often find "Best regards" "Sincerely Yours" "Yours sincerely" "Kind regards" "Best" "Cheers" etc

For the signature, also full name (first name + name) or simply your first name.