I’d start by saying... you don’t have to beat or scream at your SO to be a crappy partner who is letting their substance abuse problems mess up other people’s lives.
If his current level of drinking is making him unable to focus on conversations, that, alone, is impacting you and your relationship in a very negative way.
If he’s outgrowing his clothes and looking like a schlub, is that doing good things for your attraction to him? How is his motivation level to do anything, other than work and then get drunk on the couch? Does that make you feel happy about being in this relationship?
Frankly, a lot of what’s unattractive about heavy drinkers is the self-pitying “fuck it” attitude they often develop. Apathy is pretty repulsive.
Since there’s always an ready escape from ordinary feelings of boredom, frustration, etc... they tend to stop getting up to address even simple life stuff. And the longer they put it off, the harder it gets to deal with.
The pressure on you to pick up his slack is probably already substantial. Al anon will tell you, correctly I think, that the only way to save the relationship longterm (whether he ever cleans his act up or not) is to NOT take on any of his burdens. Hard in a longterm cohabiting situation, I’m sure!
Anyway, those are the reasons I have.
I’d also point out that there are a lot of longterm heath problems that come from that level of boozing: liver disease, alcoholic neuropathy, cancer, seizures... how do you feel about potentially becoming a caretaker for someone who has ruined his own health and made your life unhappy for years first?