Hi, I Have A 4 Yr Old Boy Who Has

Hi, I have a 4 yr old boy who has been having problems pooping while sitting on his potty. He screams in a lot of pain when his poop comes out and it was agonizing to see him in so much pain. Then he said, there's still poop coming out but he wants a diaper. So we placed a diaper, then he pooped againwhile standing. He did so without pain, and there was a lot of poop that came out. I am wondering if there is some obstruction. He is in pain whenever he poops on the potty, but not when he wears a diaper and when he is standing. What should we do?