Get him a loyalty cards in his name from the most feminine shops you know (like Victoria's Secret Angel Card, Intimissimi Card, Woman's Secret, Zara Woman, etc) and make him carry them with him in his wallet everywhere.
This is to remind him that he's a sissy and shops at women's stores. Every time he takes his cards out of his wallet there's a chance that his loyalty cards might be seen by someone, which could be an embarrassment. He'll also end up on these shops' mailing lists....so anyone handling the mail will know what a sissy he is. It's a great way to publically humiliate him without involving anyone else directly.
You don't need to say anything… just start signing him up in every woman's shop you go to and watch his embarrassment as the cards start to arrive and you force them into his wallet. When you sign him up, always list him as "Miss Sissy (and then his name), so that it get's printed on all his loyalty cards. If you are in a women's shop with him, remind him that he needs to get on their mailing list and ask for a loyalty card in front of the sales women, and make him fill out the form to both of your amusement.
Make sure you identify him as a girl, not a boy, on all the forms you fill out. Don't be afraid to let the staff have a good laugh in knowing that he's being trained and treated like sissy girl.