I went through a similar experience with my ex alcoholic, drug addict, sex addict, verbally abusive fiancé. She was a great person when sober, and we were genuinely happy for about 1.5 years while she was on her good behavior. After we got engaged, she stopped white-knuckling, and in the last 6 months she cheated on me while drunk, quit her job, started smoking weed full time, stopped paying her bills, put me in a financial pit, and she put my life in danger while drunk.
The one thing I got out of that relationship is that the best predictor of future behavior is past behavior. If he loves you enough, he will get into a recovery program to kick the habit for good. Most of the time, though, the addict chooses the addiction over their partner because the addiction is their first love. I think it may be a good idea to sit and wait, but be prepared that he will relapse again without any recovery. You may want to consider an ultimatum, but be prepared to stick to your guns if he chooses the addiction. If he does that then letting go may be your healthiest option. He will only get better when he genuinely wants to.
Remember, you didn't cause it, you can't cure it, and you can't control it.