Kris, don't do it. Think about it---he's doing drugs, soon enough, he might start to drink. If he abuses drugs, there's no doubt in my mind that he might start abusing alcohol. You could turn out to be another one-night stand for him, and you're too sweet to be that. People change, and for what it's worth, I know you can find someone better. He can be a charmer, but truthfully, I wouldn't get caught up in it. Do you think he'd be willing to seek a relationship with you that's not just sexual? That's a question I'd think of first.
But whatever you choose, I trust you would've thought long and hard about it.
I'd leave him. Even if you knew him way back when I wouldn't trust him now. If you do end up doing something with him you are most likely just going to get hurt, If that's all he wants to do then he's most likely just using you. He's not a little kid anymore.