Hi OP, I don't know much about drugs and I'm not as knowledgeable as people who have posted on this thread, so I can't really help you with the scientific knowledge etc.
However, I have experience in caring for loved ones who have addiction, it takes it's toll, and I know people who have also experienced the same.
Honestly, I'd suggest talking to him first and airing your views and opinions, if he changes or relaxes up a bit, then fine. But, if he doesn't and he continues to depend on you then leave him, he won't change too soon and he'll continue to drag you down. You're 17, you should be young and free, you don't have an obligation to deal with his rubbish, focus on yourself and your work and move on.
That's all I can say.
Best wishes, and just make yourself happy.