Drug is an addiction, and as long as the addicted person believes there's nothing wrong with it, they won't let you help them. Once he realizes in his own mind it's a problem, he has the potential to overcome it and become a better person without drugs. The hard part is to get him to see it as the problem if you decide to stay to help him.
I feel like, if you stay, and he doesn't seek help, the drug addiction will wedge between both of you, and you will grow resentment toward him.
Now, if he does seek help with his addiction because he sees it as a problem, it won't be an easy ride either. I imagine it'd be a really really bumpy road with maybe relapses and re-tries on his part several times. It probably will require a lot of patience from you.
If you decide to leave, you will be relieved on your part because you're no longer around toxic environment. You'll be able to heal and move on to a healthier relationships. For him, he will be hurt deeply, and he may or may not seek help once you leave.
Just look at the situations realistically at what might happen if you leave/or stay and what you can and cannot handle.
I hope it'll work out for the best.