S.D.
Can you tell he's about to poop or does he not stop what he's doing until after he's done? If you can tell he's going to poop, I'd just pop him on the potty. If he doesn't like the dirty diaper, he'll probably happily start using the potty. If he cries while he goes there's something going on there. Or maybe if you wait a couple days it will pass.
Make sure he's hydrated so you know it's not constipation.