Welcome,
I think the advice in this thread is absolutely golden. (And, when I say "advice" I really mean the collective experience, strength, and hope expressed.) Nobody is saying you must "walk away" from him. That's something you will have to decide for yourself in due time. BUT, as has been pointed out, once you are legally tied to someone it isn't simple to undo it. Divorce is a much bigger step than not getting married. It's harder to do, more expensive to do. And by then, all kinds of damage may have been done.
I am going to give you a hopeful story. I was very close to (and hoping to marry) my b/f I met in college. He was an alcoholic. After we left school, it got worse because he started drinking even more. It soon became obvious that even when there were important reasons not to drink, he could not stop. After many arguments/discussions, a friend at work gave me AA's Big Book. I read it, and learned a LOT about the nature of the disease. I gave the book to him, and he read it too, recognized some of himself in it, but was sure he could manage it himself. When that didn't work, he agreed to go to one AA meeting. He thought it was OK, the people were nice, but still thought he could manage on his own. When that didn't work, I was at the end of my rope and told him I needed a long break from the relationship--that he had to figure out what he wanted, but that I didn't think I could continue to deal with the drinking.
During our brief separation, he went to AA on his own, put down the booze, and never picked it up again. He celebrated 31 years of sobriety in January. We later divorced for other reasons, but he was a great husband, a great Dad to our kids, and we are still good friends.
So. It IS possible for people to recover, but they have to be ready to do that. His is kind of an unusual story (as they say in the weight-loss ads, "results not typical"). We got married after he had been sober for a year and had a solid grounding in AA and a good support system that didn't depend on me.
I don't know what would have happened if we had gotten married while he was still drinking. My guess is I would have become more and more enmeshed and despairing and that he might have taken a lot longer to conclude that he needed and wanted to stop.
I would encourage you to postpone the wedding indefinitely. It's a hard thing to do, but extricating yourself from an alcoholic marriage is a lot tougher.