Tell him what you just told us. Things are getting stranger and stranger and you feel it is time to end the relationship. You have literally no control over whether he relapses afterward or seeks recovery. You can wish him the best and tell him you hope he finds happiness, whatever that means for him. I don't think there is a "right" way to point out the issues you have with how another person is behaving without coming across as judgmental, which I don't think is what you want.
In our relationships, the best we can hope for is that we are with someone who we can accept for exactly who they are right now and who can do the same for us. If that level of acceptance isn't there for whatever reason, it's not a crime. It's nothing to feel bad about. It just isn't. Or it's bad timing. The most respectful thing to do is move along so everyone can be available when someone comes along who can take us as we are, or so that we can get ourselves into a better place for being in a relationship with anyone. I think you probably already understand that, though.