Check with a attorney (ok). One thing you will need to do is type up a letter, make copies and mail it to him Certified - where he has to sign it. You have to give him 30 days to move out, but it has to be stated in the letter, also state that if he does not pay the back rent you will take him to small claims, to get your money. Too bad you do not have a signed contract - but he is living there scott free and that is not just. I would see if you could get legal advice for free (do you belong to a union at work- that offers free legal advice. If not, you may have to find an attorney (hopefully the first consulatation is free) and get some legal advice. Good luck to you.
You can also do a web search for "free evection rent notice"
http://www.ezlandlordforms.com/documents/late_notices/
http://www.formsguru.com/forms/legal-forms/notpayqu.html
http://www.mojolaw.com/forms/rt14f
Good Luck to you.