Why is my toddler doing this?

He’s 3.5 and been potty trained since right before he turned 3 and all of a sudden he will not go #2 in the potty anymore he was doing it perfectly fine, now we have multiple “accidents” a day he apologizes and says it was an accident but he’s clearly pushing and doing it on purpose and I can’t figure out why he’s all of a sudden too scared to go poop in the potty? He’s ruined so many underwear I’m stumped on what to do..??? We have started putting pull ups back on him but it don’t seem to help at all so I stopped that already. Help?!