How much history do you have with this man? 2 years?, 5years? 10 years?
If you marry this guy, you fancy spending the next 10, 20 or 30 years together, the way it is now.
You have the rest of your life to be happy, living with an alcoholic, or at the very least someone with a drinking problem. I call making a tough decision NOW, to tackle this and make sure it's solved NOW is an investment, not a few years you have spent rubbing along with him.
I know I was guilty of this in the past, the not wanting to throw away the investment I had made in the relationship I was in, but it made me miserable, worse than suicidal; I was forced to survive abuse and humiliation every day, day after day after day, and knew that there wasn't a way out.
You have a choice, you HAVE to live the best you can, and so does he. If he can't sort himself out, please don't allow him to drag you down with him. He has to stop for himself, and I sadly don't think this is going to happen any time soon.
Your comments of If I told him I'd leave, he'd stop.... But you haven't asked him have you? Perhaps you don't want to explicitly ask him, because you know he'd either refuse, or he'd fail. You think he'd do it for you, but you are not sure. As we say here time and time again. ACTIONS not WORDS, you need to decide what you need and stick to it. Don't ever sell yourself short.
My abusive X, I asked him time and time again to just be nicer, not treat me equally, or cherish me as is actually my right, but just stop being nasty. He couldn't, didn't think he had to, didn't want to give up the power.
Your P is addicted to alcohol, or at the very least USING alcohol rather than just enjoying it. In some ways, he has a harder battle than my ex would have had,
You need to lay down the law, I rather think, you may have to leave for him to sort himself out. If he does, great, you carry on and all will be great. If he doesn't, then you have prevented yourself from being dragged down that hideous road with him.