I can only offer you this advice. Separate the idea of whether you want to leave him from his drug problem. If you take the drugs away, would you be happy.
If the answer is yes. Then try to work with him. Ask him when he’s not high, does he honestly want to stop or is he happy to carry on as he is. If you’re going to stay and help him. Try to sit down with him and make a plan. But I mean a real detailled one. Work out how much help he needs and how much control he is willing to give you. And the biggest thing, make him feel comfortable with telling you when he does do it. If he feels like he is being judged he will just hide it