hi KittieKat....I am an alcoholic male who drank for 27 years and quit 9 weeks ago.
I think you need to focus on what you can do here, not on what he can or might do. The reason is that alcoholics will do what their addiction demands of them not what you would like. The alcohol has a far stronger persuasive voice in his head now than you do so you would be just wasting your breath.
I think you can do the following independently of him:
1) spend as much time as you can reading through this site. You will see your story repeated over and over and over again. Yours is a well trodden path
2) Attend alanon meetings and research alcoholism so that you can appreciate the enemy and realise exactly what you are dealing with. Most people understand remarkably little about this disease
3) Realise that inside his head now is an alcoholic fog that is not rational. He is literally not thinking or seeing straight which explains the roller coaster emotions. His niceness when sober is probably laid on thick to deflect from the drinking. You cannot win an argument against a fogged up alcoholic mind and you cannot make sense of it
4) Realise that he will not recover until he makes sobriety the number one priority in his life and if he does not then the problems he faces now will be tiny compared to what is coming down the road....this is because alcohol is a progressive disease.
I met my wife seven years ago. The entire time I met her I was an alcoholic. I proposed to her when I was drunk. I held and still hold a great job, I run charities. Inside I was a gibbering out of control mess...a car crash waiting to happen. Then bad things started to happen. I was run over when drunk, my best friend died from the drink and I got a DUI. This is what happens when alcoholism is left unchecked. My wife knew about all of these things. I hid most of my drinking from her and 10 weeks ago when I told her that I was an alcoholic and that my drinking was killing me her reaction was "no its not, you are not that bad". I was on my hands and knees in tears at that point. It was my rock bottom. Then she didn't want me to attend AA in case I saw people worse than me. During these years I drank most days and drank to blackout every single time I touched a drop.
This disease can be really insidious and sly, but I kid you not, its as destructive as any drug out there and right now it has your fiancé and its not giving him back until he hits his rock bottom. So if you do choose to stay with him expect rock bottom and expect it to drag you with him. Please protect yourself and then let him protect himself. As totally painful and ruthless as it sounds he needs to sort himself out.