Mnicot, sorry you were cheated on several times.
My first concern is you...regardless of the situation, he has slept with several people, you have no guarantee they used protection and potentially have been exposed to goodness knows what diseases.
Please get tested, no matter what he says. A year is not a long time to show he is worthy of trusting and his past behaviours, continual cheating and relapses aren't a good indicator.
Secondly...he needs to get well. I don't think his headspace sounds anywhere near being able to operate in any relationship yet. I'm sorry to say, any woman is a distraction from the real work he should be doing, which is getting his backside out of drugs.
You deserve better. I don't care who he is. If he cares for you, as much as you do him, he will come back when he is well.
Best of luck and I'm so very sorry for what is no doubt a painful situation.