Speaking as someone who suffered from severe depression at one point, I think you should wait until after his operation to see if there's improvement when he gets help. Yes, that seems like months away now but it'll come faster than you think and he could recover. Try to see it this way, you feel he's dragging you down but at the moment you're pulling him up, whether it seems that way or not. Do spend time with other people so that you're not only around him but keep in mind that you being in his life will help him recover.
It seems as though you care about one another a lot, I think you just have to hang on during this rough period. You don't want to lose someone special just because they're ill. He can't control how he feels and as hard as it is for you, it's harder on him.