Screaming on the potty

My son, who is 32 months, has been potty training for a while and we've come to the point where if he has to poop it's a whole ordeal. He'll tell me he has to go potty so we'll go but he looks like he's going and then doesn't, then he gets off the toilet and the cycle repeats for up to an hour. When he finally does start actually going #2 he starts to scream and cry like it hurts ( it's not hard so it shouldn't I would think?) until a few minutes after getting off the toilet. I'm just curious if anyone else has experienced this and if they have any advice on it?