It takes some serious balls to tell a parent in need no, but sometimes that's what's got to happen. It is possible though.
I know this sounds cold, but you've got to think of him not as your father, but as your equal. As a friend per se. Would you let a friend whom you parted ways with on bad terms come back 12 years later as an alchoholic and live with you for 20 some-odd years (maybe more)?
He's an adult, he made mistakes, and he has to face them. You can certainly help him, but as far as living with you goes, HELL NO.
It will definately strain your relationship with your SO at the very least. I can gurantee that. Not to mention he extra cost/stress/time/etc it will absolutely ruin your life.
Not to mention the fact that he'll probably be drunk half-the time.
As for what to say, there's no nice way to say it. Just get straight at it. Tell him no and tell him why. And be solid. It doesn't matter if he promises to do AA, pills, whatever. It's your life, your decision is final.
Think about it this way: He's allowing his individual problems to severley harm your quality of life, and he probably knows he's doing this at some level. IMO for this alone he doesn't even deserve to call himself a father except in the most biological sense.
Parents don't intentionally let individual problems they can fix negativly effect their children. Period. If they do, they are weak, moronic, and never deserved to have kids in the first place.