It's tough to try to make things better for people for a number of reasons.  While there is the possibility that you could have done something to prevent the eventual outcome, there is also the possibility that this was going to happen regardless of your actions.  The world can be a tricky place sometimes, it's best to realize our limited perspectives and appreciate things as they are, even if they are depressing. 

As a counselor, I can tell you that he may well have done the same thing if he was living with his mother.  The choice was his. The demons that drove him took hold long before he stepped into your courtroom.  The decision of the court did not fatalistically predetermin the outcome for his life.  Sometimes decisions can create a greater or lesser likelyhood of a give outcome, but nothing is set in stone.  Most clients I've seen rationalize the case for their own self destruction regardles of their exterior circumstance, particularly when drugs are envolved.

Go easy on yourself.  You gave him the best opportunity you could at the time. Once they walk out of the courtroom, it is out of your control.  It could be it was the best decision until some other intervening event changed the mix.  People's lives don't stay on hold. They are constantly changing in ways you can't control and sometimes never know about. 

Breathe easy on this one. You probably did the best anyone could do.