Almost 4-year-old refuses to poop in the potty
Help with getting almost 4-year-old to poop in the potty! He's been pee trained for months (except for overnight) and he tells me when he needs to poop by saying, "I need to poop in diaper." So he goes and gets a pullup and we go in the bathroom, he dies his business standing up in the corner, he lays down to get cleaned up and we go about our day. He's never had a poop accident in his underwear, so clearly he has the muscle control. I've tried all kinds of incentives like candy, sticker charts, etc but he just doesn't care about them. He goes once a day, most days, and his poop isn't hard or pebbly so I don't think it's a constipation issue and he doesn't act like he's in pain when he's going, so I don't think it's a discomfort thing. So it's clearly a mental thing, but I can't get him to tell me why he doesn't want to poop in the potty. And if I even try to get him to just sit on the potty while he poops with the pullup on, he immediately and adamantly refuses. Any advice or insights would be appreciated!