My 5 Year old keeps back tracking

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I'm growing frustrated but am staying as calm with my son as possible, for some reason though he will not poop in the potty anymore. He turned 5 on Halloween. I've tried everything and we've been at this for weeks, rewards, taking games away if he has an accident why playing, you name it. I've talked to the pediatrician and his advice was 20 mins after every meal put him on the toilet and make him try, which I've done, but he waits until the pants are back up and then goes or goes in the middle of the night. I feel lost and I feel like I must be doing something wrong, he shouldn't be having accidents anymore and he was trained. Any advice? I try to explain that he's a big boy and big kids go on the potty but he argues with me.