Perhaps, because he IS wired differently, and experiences life from a very different perspective. He's a square peg in a world of round holes, and he just doesn't fit... even when he "seems" to be doing the right things, life never has that "flow" that makes things move forward. He's always tripping on something? Maybe?
Doesn't excuse the drug use. That's a crutch that makes things worse. But might be part of how he got there. One of the interesting things about drug culture is that it doesn't matter if you're different, as long as you're using, you're "in". It's the one group that is willing to take in many of our differently-wired kids.