You could try making him write some phrase that means something to the both of you...if there is a nickname you call him that is particularly humiliating, have him write something along the lines of "I am Mistressfitzy's __________" somewhere on his body that will be covered by clothes and then send him out to run an errand. you can up the ante on this one by making making him cum in his pants and not clean, or wear a butt plug (something like that) to add to the humiliation. You and he will be the only ones who know he has been labeled as belonging to you, but he will still be carrying that mark and walking around in public.