Look, ive replied to your posts before, and you are obviously very very young, and probably think this guy is the best thing thats ever happenned to you, you have already answered yourself by saying 'I know he's cheating' so whats the dilemma? You have to dump him, and NOW, especially if he's some sort of druggie!!! Do yourself a favour and dont get messed up in all this, for gods sake, you'll end up pregnant and then your young life will be over, saddelled with a baby (well, 2 if you count the boyfriend!)
Sorry its not want you want to hear probably? but i bet all your replies are along these lines. Good luck, but get out now! xx