Renting to my college student child with roommates

[Heidi M.]

We recently purchased a condo near the university my son attends so he can avoid paying the rediculous rents in the area.  The agreement is that he will get a roommate and cover all the expenses until our other daughter moves in (when she's ready to leave home).  We are obviously not looking to make money off of him but plan to keep the unit and rent it out in the future when all of our children are done with school.  We would like for him to get a roommate to help with the expenses and are looking for advice.

1.  How do we structure the lease?

2.  How to handle the security deposit from the roommate?  We were not planning to make our son pay the security deposit.  He is a very responsible and clean kid.  We are confident he will take care of the place.

3.  How to handle returning the security deposit to the roommate when he moves out and my son is still there?