Boys and the potty!?

So my son will be 3 in March and so far we can get him to pee on the potty all day long. But when he has to poop he will tell me he wants a diaper. I cannot for the life of me get him to poop on the potty. I put training underwear on him and he acted like it was a diaper since it’s padded with cotton and pooped in it. I sit him on the potty when I know he has to poop but he will just sit there for the 20-30 minutes and not go. I will finally just take him off and tell him to let me know when he has to go, but he will just continue to poop in his pants. Does anyone have any tips or tricks to get him to poop on the potty.

He has his own potty and we also have a seat for the actual toilet seat.