If I were in your shoes, I would talk to a lawyer. I would get good advice. I do NOT know the laws in your state (and don't know if you really live in VT). I would want to know if your son can be labled a sex offender by the authorities for having sex with a 15 year old. I would want to know if the girl's parents can have him arrested. They might not always think he's a nice boy.
If it would work and if your lawyer agrees with it, I'd want a notarized letter from her parents stating that they are okay with your son having a relationship with their daughter. I have no idea what the lawyer would say about this, but I think it's worth asking him or her.
Honestly, if she's considered underage, I'd keep trying to keep him living in your home. At the point she is no longer underage, and your lawyer agrees that he can't be charged with having sex with an underage girl, I would tell him that he needs to find another place to live until he can decide that he wants to be a college student or not. Your son needs to grow up and letting him stay at home on "easy street" is not the answer.
Either he helps in the house, doing chores that help with the family, lets you know where he is and what he's doing, or he finds another place to live. Someone will rent him a room and he can pay rent from his part time job. And he can pay his own insurance - you need to take him off of yours. If he has a wreck on your insurance, you will be liable for it.
If he lives on his own, with his own address and paying his own bills for a full year, and he will not be considered your dependent. He can get financial aid for going to college. He'll have money for a place of his own and will have no one to yell and complain to if he doesn't make good enough grades to keep his financial aid. That'll make him grow up.
You need to show him tough love after you find out what's what legally. Get a lawyer and work this through.