Why doesn't my little boy want to poop in the potty. He pees in the potty but never wants to go poop…

Why doesn't my little boy want to poop in the potty. He pees in the potty but never wants to go poop in the potty. He did good all day with peeing but about 10 minutes ago he pooped in his underwear. Plus he just went pee right before he had his accident so I don't get it. Anyone have ideas and how to get him to go poop in the potty.