Your fiance is in advanced alcoholism, which means the odds of solid recovery are not good. If it were me, I would not marry him and definitely make certain not have a child with him. Too often addicts subconsciously use formal obligations as a way to keep their enabler trapped in the relationship and make it safer to relapse.
If you want to keep trying with him, just don't make it legal so it will be easier to leave.
I'm sorry...these are harsh words but they're based on sad experience, both of mine and that of a lot of other people.