Mark - I think you handled that situation in the best way possible. Now he knows he can reach out to you when he's in a bad spot like that, and he can trust you to receive him with respect and reasonable concern.
You can't control everything he's going to do, even though we all know your life experience has taught you exactly what he should not do. I worry about the same thing with my little girl, and believe me, I wish I could just tell her "hey, don't ever drink, you might be an alcoholic like me and half the other people in my family," and she'd say "ok mom" and never so much as use mouthwash from that day forward. But I know she's going to drink no matter what I say. Not to spite me, but because young people test the culture they live in to see where they fit. Alcohol is a huge part of our culture. As much as I don't want her to drink EVER, I also don't want her to approach a large part of society automatically feeling like an outsider. I will definitely have the conversation with her when she's old enough, and I do intend to tell her the truth about my alcoholism, rehab, recovery, and everything, I want her to know. But she's going to do what she's going to do, and I accept that because I also want her to be brave and open minded. All I can do is educate her and make sure she knows I don't want her to put herself in danger, but I won't turn her away if she needs help. From there I suppose the conversation will be whatever it needs to be.
That's how I view it now. That may be harder to stick to when she's packing for college!
It sounds like you're doing an excellent job, and I am so impressed that he reached out to you at a vulnerable moment like that. He loves and trusts you.