hi-
well, you say he's an active alcholic, not in recovery but you want to continue your relationship with him.
so, it appears your decision is to maintain the relationship, even though he's drinking.
in that case, i'm not sure that i see the issue with the new year's party. you accept his drinking, you both want to go, so why not go?
if it was me, i would arrange to take my own car or organize a ride home. i also would also be clear that i was going home alone and he was going to his mother's house.
that way, you can go to the party but you don't have to deal with the drunk at the end of the night. you leave alone and leave him to deal with the consequences of his drinking and how he's going to get home.
naive