First off, take a step back: the situation between your son and his father is not your doing, and any guilt you feel about it is unwarranted. Your husband seems to have issues controlling anger, and should look to start counseling; either with you, or alone.
Failing as a parent takes alot more than finding out your kid is dabbling in drugs. There's been a breakdown in communication, and it seems like he's self-soothing. Again, blaming yourself or your husband isn't necessary, because your son is making these choices.
Take your son to a neutral place: go to the movies/dinner, go walk through the park with him, something where the "spotlight" isn't being put on him an his behaviour- that always makes people apprehensive about sharing important information. Ask him why he does it, what attracts him to it, how it makes him feel. Talk about finding safe and healthy alternatives to his drug use. I believe getting to this early will be a huge step in the right direction, but remember: DON'T ATTACK HIM. When someone is attacked, they automatically fight back or shut down. That will get you nowhere. Try to be stern, not harsh.
If (later down the road) you find out he's still engaging in the drug use, more harsh discipline will be nesessary. Nothing too dramatic right now, it's surely a stressful time for him.
I don't believe your son is doing this to "rebel", merely to help him deal with the pain and tension he must be dealing with due to his relationship with his father.
Once more: YOU'RE NOT A BAD PARENT. You just have some obstacles to overcome in the future. Good luck.
