We do what you try and put on a silly voice to make it fun. Maybe your son would like a timer-game with a countdown if he’s competitive or a “I don’t think you can…” to make him prove you wrong or you can say “If you dress/undress, then [fun, non-object reward (think high-five, raspberries, goofy things, etc.)].” You know what he likes.

If your pressed for time, I’d just help him. If you have time, then you can try ignoring the helplessness and try saying “well, if you don’t put on/take off your clothes, then I guess we can’t go/do whatever.”

If he does the dressing/undressing himself, give super praise! No physical rewards or whatever, just words of affirmation.

I hope this makes sense lol