Help! How to break this Potty habit!

My son will be 3 on Saturday. When he’s bare bottom he knows where to go potty both pee and poop not a problem. When he wears a pull up he also knows where to pee, but he feels comfortable pooping in the pull-up. When he wears pants with nothing underneath, or underwear he’ll just pee/poop himself.

I’m not sure how to fix this problem. Why is it he is so confident using the potty when bare bottom , and with a pull-up on when it comes to peeing, but as soon as you put pants with nothing underneath or underwear on its back to the drawing board. He knows where it goes, why doesn’t he do that consistently?

Any suggestions or ideas would be fantastic!