As Barbara noted: "Ask yourself if your fiance is the man your want to share your life with as he is right now, not as what you think and hope he can become, as he is right now."
If he is not, you may end up spending a lot of time waiting and hoping for him to find and maintain sobriety and become that man that you want. As the child of an alcoholic, I wanted to add that, if you decide to have children with this man, you may force them to wait and hope with you. It is that much harder to walk away from someone once you have had a child with them, because it's not just about you anymore.
No one can tell you what to do, but your gut feelings are a very good place to start in making a decision.