Ds is about to turn 4, was quiet late potty training but when he did he got it in a fe days (3.2 ish). He's dry at night 99% of the time but for some radon for the last month he's started pooing his pants. He knows he needs to go as he stands up and starts running around holding his bum 
but just won't go on the potty or toilet. Even when I can get to him quick enough to sit him don he not go, then stand up and poo in his pants 30 seconds later. I've tried asking him what the problem is and he just says it's an accident. I've asked if he's scared or it hurts and he says no, gets really upset at the suggestion of going back to pull ups etc.
I don't shout at him when he does it but have tried to explain that it's really important that he doesn't poo in his pants. He agrees, then poos in his pants!
Any ideas? I'm starting to avoid going out 