I'd recommend you look up Flesh_Seraph's thread on the psychology of humiliation. Whilst it's not aimed at males, it's still a good basis for forming ideas.

Other than that, if he has no desire to be feminised, you can still make denigrating comments about his masculinity with no explicit suggestion that he's effeminate- reduction to an agendered state can be just as humiliating.

Or find some other angle to attack him from- for example, if he prides himself on maturity and independence, humiliate him by reducing him to a childish level, or for being unable to control his arousal in your presence. Humiliation is deeply personal, because it requires knowing what your sub's buttons are in order to press them, but finding out is surely half the fun.

Good luck!