You might be able to find low-cost rehabs in your state, if you're not too rich. Many locations do have programs that are affordable and sometimes free.
Your little darling can go to community college, if he does well he can move on to any school--not all colleges are going to excoriate a child for some weed-related stupidity in secondary school. He can also home school if you want to put your boot on his neck--there are a lot of online schools nowadays; the trick is to find a good 'un.
You should also try to get the little fart's record sealed and/or expunged, if he is young enough. A lawyer can help you there. Then, all the bad shit can kinda go away.
Hell, the MILITARY would take him with that "crime" on his record, assuming there isn't a pattern of misbehavior and he sticks to a "I was experimenting, it was only the first, second or third time I ever tried it, I was easily led by older, more sophisticated peers" excuse. He'd have to go through the waiver process, but it's not too onerous, assuming he doesn't look like a "scary" kid to the straightlaced population, with crazy hair, piercings and too many tattoos. He would get training, and be eligible for the GI Bill, which will defray a fair amount of that college expenditure. He could also take courses while he serves to get a jump on his post secondary education.
Life isn't over for him--he just fucked up. He needs to understand that, certainly, but his path will just be different, not a complete disaster. It might even be a good thing in the long run. It's better to learn the old "Actions Have Consequences" rule when you're fifteen and not fifty or more, like the Republicans do.