Have pretty pink and lace lining and labels put in all his clothing. Take all his favorite pants, shirts, sweaters, and jackets to a clothing alterations service, and ask them to place pretty pink and white lace or liner inside all of his items. Have nice label's made that say "Miss Sissy (his name)", and have them installed on all his clothes. Have the buttons changed on all his shirts to pink as a trademark , so they are the same on every shirt.
Nobody really sees the inside of his clothes except when he's taking them off or having them washed… and anybody who is exposed to them should get the hint that he's a being re-educated and treated as a sissy girl. The pink buttons will be visible, but low profile. It will be a humiliating and embarrassing reminder to him every time he puts on or takes off his clothes. ... And it's very very very cute!
Just tell him that it's so easy to get his clothes confused, you needs a better way for the maid and others to be able to sort out what's his. Tell him that the sissy labels and the pink and white lace seemed the most appropriate, and that you discussed it with the women who do the alterations and they agreed that it was the best approach.
Do this when he's away on a trip and least expects it. Make sure you have his clothes back by the time he returns, and make sure it's in every item of his clothes, so that he has no escape from it within his closet.
If any female friend's see the labels or lining in his clothes, you may want to comment on how cute you though it was for him, then ask them to tease him about it subtly and silently.
You have to be sure to have the labels and lining in all of his normal daily clothes, not just one or two pairs of something. Make it reasonably hard for him to be able to hide, so that anyone can see the lining and labels by picking up the clothes. Make sure that any new clothes he buys are immediately modified to include the same labels and lining. When he shows up to the alterations shop, the women working there should automatically know what to do, without him asking.