B.,
Is he using a potty chair or the toilet? Which ever one ,try the opposite and play it up big time , "you are a "big boy" . Sometimes children who are trained on a toilet have accidently fell in or had the feeling they were going to fall in and it makes it a little scarry for them, you can get the potty "seat" ,which fits right on the regular toilet seat to prevent them from falling in. Have you trained him to pee on the potty seat or stand up at the toilet? Sometimes that can be confusing for them too. Have you checked his BM'S too see if they were especially hard, could be it hurts when he poops ,no matter where he maybe trying to go and he just wants to avoid the pain of going. Make sure he gets plenty of fluids and fruits and veggies, it makes it a whole lot easier to go. Reward him if he does go by offering new character underwear he likes, (you might even let him pick them out) and remind him that they are big boy undies and he doesn't want to "mess them up" , so he can wear them again. With one of my children I had a similar problem and made a game of it, there's the cookie monster and then there is the "Poop Monster" who gobbles up thier poop when the toilet flushes, (sounds silly I know , but, it worked, sometimes you have to be a little creative :) ) I don't know if any of this may help , but, it's worth a try ! Best of luck to both of you, C. S.