Hi there,
I am not a parent BUT I will say that once my Dad stopped enabling me, I got better. He paid for me to go to 6 weeks of rehab and I came out and drank. He loaned me money, payed my credit card off, payed my rent etc. and I drank and drank and loaded up on Xanax. The last time I wound up in detox he said "I'm done enabling you. You need to go to rehab and you will be paying for it yourself." He distanced himself from me and did not visit me in rehab. In fact, the whole family went on a 2 week vacation to Hawaii without me. I've stayed sober since that day he let me fall flat on my face. It was the greatest gift anyone has ever given me - to let me fall on my face and pick up the pieces of my life myself. Since leaving rehab I've dealt with financial troubles and my father said "I know you've got a lot of debt, you might have to file for bankruptcy or work out a payment plan for your bills" but he did not offer to give me any money. In 10 months I got a full time job in my career field, a cute little apartment and I have a nice, sober life.
There's no right or wrong way to approach this and I certainly feel for you as your hands are tied it seems. You are absolutely right that rehabs want your money. I went to rehab 3 times (one of them I walked out of after 24 hours because I was such a brat). The sad and scary thing is that the desire for your son to be sober HAS to come from within. HE has to realize how out of control his life is and how miserable his situation is. He may be doing fine at school now but soon he won't be. Addiction is progressive. Over time it gets worse....never better. It make take him failing out of school to realize he has to stop.
What really helped me get sober and stay sober was Big Book Step Study meetings of alcoholics anonymous. These are very specific meetings that focus on the 12 steps of recovery and how to work the steps. I got a sponsor immediately out of rehab. I did and still do whatever she tells me to do no matter how insane it might seem. I am not sure where you live but Big Book Step Study meetings are big in certain areas of the country. BBSS really helped me see how much of a spoiled brat I was and have helped me turn my life around.
Also, I was diagnosed with ADD, Depression AND OCD as well. I was on loads of medications. Once I quit drinking I was able to come of all medications and I function perfectly fine.
Sending warm thoughts your way.