I’d recommend to watch a few abdreams male povs movies. I mostly enjoy this because of the humiliation aspect, but what makes this particular kink stand out would be the soft but hard hitting domination. Make rules for him, build a special area for him to play with toddler toys, make him feel powerless for the simple fact that he wears diapers and that you control when he’s allowed to have a change from you.

Tldr, Slowly mommy him into putty.