Tell him to pay the bills BEFORE he moves in. It will be a good object lesson for him.
As for talking to the collection agencies, that is the kiss of death. They are well-practiced in dealing with deadbeats and know every trick in the book. If you have to ask questions here about how to deal with this issue, you're a prime victim for their tactics (no offense intended) They'll have you for lunch.
Spend some time reading the Clark Howard material. He is an acknowledged expert. Also, some more information here:
»www.privacyrights.org/fs/fs27-de ··· coll.htm--
Nothing makes an American want to do something more than telling them they can't.