I'm very sorry to hear about your situation. My only advice is that if you issue an ultimatum, make sure it's specific ("If by May 30 you are still having two or more drinks per day on more than half the days in a week, I will go to a lawyer to begin divorce proceedings"). Vague ultimatums (cut back on drinking or I'll leave) have far too much room for interpretation. Also, you have to follow through. If you issue an ultimatum and don't carry through, in the addict's mind you've decided that their drinking isn't really that bad and so they don't have to change anything.
I gave my alcoholic ex 90 days notice. On day 89 he was still drinking and acting strangely. On day 90, I left.
Ultimately you have no control over whether he stops drinking. The only thing you can control is whether you are willing to spend more years in a marriage where one party is drinking in front of the TV every night.