Potty Training

Hello! My almost 3 year old son had no issues with pee potty training. We’ve had very few accidents. He was the one that started potty training himself- we didn’t push it. However, he doesn’t seem to have any interest in pooping on the toilet. We have tried rewards, a chart with a big toy prize, throwing his underwear out rather than cleaning them, special underwear (Blippi). We never shame, we just remind him that when he needs to go he should tell us. Not sure if this is a result of his stubbornness, or if he’s scared, or if it hurts to go? Any suggestions would be wonderful!