This is an extremely normal symptom of all of the change in his life. The more you can treat it as a normal, minor thing, the more it will just go away. If you blow up over it, and give him a lot of attention, either negative or positive, for pooping in his pants, the more he will keep doing it. Why? Because it is getting your attention. It really is that simple. When he poops his pants, clean him up and move on to something else with as little fanfare as humanly possible. I know you want to yell and scream and give him a time out or whatever your punishment is, but restrain yourself. Go ahead and go into your room in private and scream into a pillow if you need to. been there done that when my kids were little. But the more attention you give this bid for attention, the longer you will have to deal with it.
It IS normal for him to act out and need more attention. It IS normal for him to regress in some ways after such big changes in his universe. A man in his home and a big move are HUGE changes and then a new school? WOW! His entire world has changed. It really is a wonder that he isn't doing more to get your attention, rather than just pooping in his britches occasionally. He could be wetting them constantly too. Or drawing on the walls with the poop. Or any of 1000 other things. I have a cousin that wasn't well adjusted, so I could probably name some of the nastier things he could do. Like finding ways to get hurt, then telling the hospital staff that you did it to him. Trust me, that will upend your life faster and longer than any poopy pants ever could. And yes, my cousin did this four times to his mother. Luckily my brother and I were ALWAYS with him and we always said he was lying through his teeth. As did my parents. My cousin was a BAD liar at that age, so it was easy to see he was lying, which helped my aunt. But your son could choose from a LOT of more difficult to handle attention getting behavior. Things that could truly harm your family. So as icky and awful as this is, it really could be worse.
Please be patient with him. Please don't send him to school in pullups as this would embarrass him. I know parents who have done this and the kids got teased horribly for many years. Kids are awful to each other. Give your son time and patience when his pants are clean, praise for pooping in the potty, even rewards for pooping in the potty (candy and ice cream or whatever he likes but rarely gets are awesome motivators for little kids). But don't give him a big blowup for pooping outside of the potty. Give as little attention to that as possible so that you don't reward that.