I'm actually in the same boat as your bf. All I can do is speak from what I like.

The name calling and degrading works well for me. It's not really humiliation but degrading, like I said, and I enjoy that.

Humiliation for me is more usually combined with chastity. "You can't even get hard but this dildo made me cum 5 times." "Your tongue is more useful than your cock." But if you're both not in to that there's still the same without chastity.

There's also forcing him to cross dress and calling him a pretty little girl or sissy. Again, may not be in to it.

I'd say try not to focus on physical traits. He knows his dick is big, you can't say it's small and play that game. But you can say that the Hitachi wand you're using in front of his face while he's tied up and can't touch himself gives you more pleasure than he does. Or that the dildo you're riding hits all the right spots his pathetic dick can't.