Sounds heartbreaking, but you're doing great in protecting yourself.
We tried a contract with my daughter, but, frankly, I don't think they usually work, but it may be worth a try.
Pot is going to be th e next big problem for all of us. I think, sonner rather than later, it will be legal and we will all have to deal with it differently, just like we do with alcohol. I can't tell you what to do. I can share what I'd do and that's it. I did have a daughter who used drugs, but it was way beyond pot. If she had only smoked pot, and had not done it on my premesis or talked about it to me, I probably would not have done anything unless her behavior became horrible.
My pot rules now go like this: I don't want to know and you can't smoke in my house. Not a cigarette and not weed. Nothing. It's my house and my rules and even if you are smoking cigarettes and do it inside, you are at risk for being asked to leave as I don't like smoke in my lungs and you will have to do it elsewhere. We have a "No Smoking" sign in our living room, but we don't know anyone who smokes (not close anyway).
I don't blame you for not paying that much money for college. My daughter has had to get grants and loans but she is using community college. If your son is really motivated to do college, he has many choices. Even if he had been a fantastic football player in high school, the chances are not good, if he has not been recruited, that he would have made the team in a four year college. It's just one of those very rare things saved for only the very, very best and luckiest in the country.
How is your son behaving outside of the pot? One bad thing about pot is that while we usually know about the pot (kids are pretty forthcoming about using pot), they often are also using other drugs that are quite dangerous. They are not forthcoming about that. That's why I asked how he is behaving. Is he respectful? Does he help around the house? Does he come home on time? Does he pay his own bills or some rent?
You have to decide what YOU can tolerate and act accordingly. Each of us and our children are individuals. It is your house and you know what is best for you and, yes, YOU MATTER. You also know your son and what he's doing and how he's behaving and if he seems to being going the wrong way in many areas of life.
Hugs for your hurting heart and keep posting.