Ok, so we managed to toilet train DS pretty quickly in the new year. He turned 3 in Dec. Prior to that he'd been sitting on the toilet to wee from time to time, but wasn't really all that interested. Suddenly in the new year he was very keen, we put him in undies and within 10 days he was using the toilet for all wees and poos and very pleased with himself. He was in nappies for sleeps and overnight, but otherwise managing well with undies and only wetting his pants if he got preoccupied with something else. All good. And then I went back to work and he went from 2 days to 5 days a week at daycare and moved up to the next room. Since then, he has pooed his pants pretty much every day, with only a few on the toilet after much coaxing. If we ask if he needs to go - and it will be very obvious - he says no, gets angry, runs away, then poos his pants and comes and tells us. Every day I collect him from daycare, there are dirty pants in his bag. We've tried talking to him about it, telling him all his friends poo on the toilet, getting him to promise he will too. He always promises, then the next day he poos his pants again. We had given him a prize when he first mastered the toilet poo - a scooter - and have now taken it back, and he couldn't care less. He's not interested in rewards of any kind. He's an extremely stubborn, challenging kid, and he knows when he needs to poo and how to do it on the toilet, he's just refusing to do it.
What should we do?