4 year old repeatedly pooping in pants

My 4 year old son has been potty training since 2 and 1/2. It’s been a real struggle. He picked up peeing in the potty pretty quickly and goes on his own throughout the day and at pre-k with no issues and rarely has an accident. He also was going #2 in the potty when rewards were being given but after a few weeks he did not care about the rewards and would not go to the bathroom to go #2, he would just go in his underwear. I have waited and waited thinking surely he will catch on because I work with him every single day. He just turned 4 and is still pooping in his pants. It’s multiple times a day because he will only a little and then i will clean him and he will just do it again. I usually can get him to sit on the potty but sometimes he will just sit there for 20 min and do nothing then immediately poop his pants once he’s dressed again. I have no idea how to fix this. He’s at the age now he understands what I’m saying but he’s extremely defiant and even screams no and refuses to listen to me when there’s no reward being given for going to the potty. What else can I do??