Prescribe forced exercise regime with targets… sit-ups with butt plug, push-ups with floor mounted dildo; in feminine exercise clothing of course; buy him a DVD with a female oriented exercise routine. Enforce his training with a whip and clear instructions.
Enroll your sissy in female group exercise class at the gym. Seek out a class like Pilates or aerobics that is normally 100% feminine, then send him to the class and make him participate.
This is a sort of public humiliation, although the other women may not realize it. He will be emasculated by having to do feminine exercise under your strict female authority.
You should tell him that you want him to go to this class because it will help you control his exercise habits to be more feminine. Tell him he's not man enough to do the men's classes, so you think he'll fit in just fine in the women's classes.
Get him some slightly feminine details to wear to when he goes to the classes, like pink socks, or a workout shirt with pink stripes, etc. You should enroll him in a class with several young attractive girls, not old ladies, so that he really feels intimidated.
Do this as a surprise, and insist that you will punish him if he doesn't comply. Ask for a schedule of classes from your gym and circle the ones he would be allowed and encouraged to attend. Then, make him reserve a spot in the class.
This can be your own personal way of flirting with his public humiliation without directly involving anyone else.