The 3 c's are big around here. You didn't cause it, you can't control it and you can't cure it.
So, ultimatums don't work. He won't get better until he is ready to get better, simple as that. You have NO control over the situation.
What you can do is start going to al-anon and begin to work your own recovery. You can decide if you want to stay or not but that choice is up to you. Any ultimatum will most likely turn into some kind of drama-fest where he agree, resent you for it and start drinking again or get really angry and keep drinking. Either way it will be your fault. Been there and done that with both of them.
Your friend,