Al-anon was a lifesaver for me during a very dark period of my life.
I saw huge red flags when I married my ex. But being young and stupid, I trampled them. I attempted to separate from my ex before we had kids.... and he promised to change. He went to therapy, even and things got better, for a time. We bought a house and had 2 kids.
Things really didn't spiral until I had my 2nd child. And I was left with no other choice but to get divorced, because I couldn't leave my children alone with him. And no, alcohol was not his "drug of choice" (that would be pot, and porn, and food, and prescription meds too, and whatever else he could find to be compulsive about) but I found my serenity at al-anon, and they welcomed me with open arms.
Now I'm a single mom of two young children, and I am stuck dealing with an angry person until they are grown. My 8 year old told me she doesn't like spending time with Daddy, she doesn't want him taking her to her soccer games because he "judges her". So now not only am I a single mom, but I've saddled two beautiful girls with a dad who isn't a real person, who is so angry and short-sighted. No, he may not be "active" right now but he doesn't have a program, either. Even if your fiance' were to stop drinking, chances are he would just cycle to something else.
LISTEN TO YOUR INNER VOICE. I didn't. And don't get me wrong, I love my kids... and I'm so glad to have them... but they are always feeling sad because they miss their dad. But then when they're with him they say all he does is sleep and put them in front of the TV and it's boring. Breaks my heart, actually.
PLEASE at the very least POSTPONE it if you are having ANY DOUBTS. Divorce ruined me financially, and now our children suffer.