Well I think he should certainly be marked as a cuck given that adorable 'Cuck Walk' art, so maybe a nice horned male symbol with cuck written in it above his crotch for all to see!

Now the question is what humiliating tatoo would best suit the 'diapered' sissy fun. "If you can read this I need a diaper"? "Diaper me!" Or maybe just the art/outline of a big poofy diaper right on his butt? Decisions decisions.

yeah cuck is something for sure ;P

A pink heart on the right side of her ass XD

plausible

A tattoo of a butt which has a butt shaped tattoo on it and it's right on his butt lol

(Let me know if you get the reference)

buttception

Needs a succubus tramp stamp. ❤️💦

hehe fun idea. but what would the stamp power be ;P

Maybe something wam themed like 'slime slut' or 'gunged goon' on the shoulder?