Very normal, especially with all the changes. You say you are at your wits end. I'm not sure what that means considering that you say you feel awful for him.
Anyway, don't ask him if he has had a toileting accident so that he doesn't lie to you about it. Just tell him to come in the bathroom and clean up. If he poops in his pants, he has to clean his underwear into the toilet to get most of the poop into the toilet. Don't do it for him. Don't fuss at him. Just insist that he do it since it is his poop. Have him carry his underwear and pants to the washer and put them in the washer. Help him with the soap and turning on the washer. This helps him OWN his problem rather than giving it to someone else.
Then put him in the bathtub. If he balks, tell him that he has to get the germies off of his bottom so that he doesn't get sick. Don't argue or fuss. Just be very matter-of-fact.
All of this costs him time and trouble. He has to stop everything he is doing and spend time on all of this. If he has been outside playing, perhaps he doesn't get to go back out because "now it's late". You don't TALK about the consequences of pooping his pants. You just make excuses why he doesn't get to do what was fun anymore. And you DON'T sit him down in front of the TV or iPad after he does this. Instead, he does something boring.
He has to decide not to do this. It's up to him. Don't be at your wits end. It isn't the end of the world. He will have his own consequences. If he does it at school, the other kids will come down on him. (Make sure he has clean clothes at school because they will call you to bring them if they don't have any to change him into.) You can stop taking him places for a while (no playdates) if he's doing it somewhere else.
AND, you can take him to the bathroom every couple of hours while transitioning him to another activity. If he balks, just say "Well, I can't get the trains out until you've gone to the bathroom." And expect him to just do it. Keep your voice neutral and stick to your guns. You probably have another 6 months of this. Moving is really hard for some kids.