Get your sissy a cute little Tramp Stamp. A Tramp Stamp is what most people call the sexy little tattoo just above a woman's ass and visible above the line of her jeans. I don't suggest an actual Tattoo, but perhaps a nice temporary tattoo that says "sissy" or simply has a nice feminine flowery design to it. If he tan's with the temporary tattoo, the mark will stay for weeks at least.
Tramp Stamp's can be sexy, but are surely very feminine, so forcing him have a tramp stamp will be a firm mark that he's being feminized. It's a source of humiliation that can be called upon at any moment, as you can easily raise the back of his shirt to show anyone of your girlfriends what a sissy he is. It will also serve as a constant reminder to him that you are forcing his feminization.
You should tell him that you find the tramp stamps very sexy and as a good little bitch he's become you want him to have one. You can strap him down naked to a bench and blindfold him as you tell him this and then simply start applying a temporary tattoo just above his ass. Every time you see it, you should remind him of what a slut he looks like and that it make you want pull down his pants and fuck him from behind with a strapon. Tell him it will look really sexy in the photos that you will take from behind him while he's on his knees and sucking a man's cock for the first time.
Pick out a design that will look nice just above his panty-line and place it so that it can't be covered up when he has pants on, only when he has a shirt on. Pick a feminine design which really enforces his position as a sissy…. Perhaps a silhouette of femdom scene, or a women with a strapon, or a poetically written "sissy boy (or girl)".
Do this after he's had his body hair completely removed. You should forcefully do this, so pick a time when you can have him strapped down and tied up with his ass in the air and can't move.
You could do this at home alone with him with the room dark and candle lit, while you are dressed like a stunning dominatrix. You could have friends over to observe and help in his "branding", or you could also do this at a fetish party in a very public ceremony with people looking on.
You should take the time to pick out a nice and cleaver design, and a method that make it semi-permanent… like a long-lasting temporary tattoo. Don't try to get him involved in helping you apply it, plan it, or select it. You need to do this with force, as a surprise, after he's not able to move and he has his naked ass in the air ready to be branded.