Casey I do feel for your son..."big pain big addiction" is something I understand. I come from that place too...
There's just so many 'reasons' why people start drinking or using drugs. Usually as youngsters or teens it's just to fit in or to have fun. That's how it was with me, but i know that once I discovered alcohol's painkilling effects, that's kind of all she wrote for me.
I do believe you are doing right by your son by making him deal with the consequences of his own actions. So many parents enable their kids to keep on using, simply by handling all the bad things for them...Or trying to ignore what's happening, thinking they're just going through a phase or something.
I have quit drinking/drugging before but it was always because someone in my life thought I needed to & put me in rehab, then I'd always go right back to it as soon as I got out. This time (and it's still really new right now)I'm not doing it for anyone but me. I've not gotten very far yet, but the big thing I have noticed is how different it feels that I want it for myself this time.