V.W.
Sounds like he has regressed somewhat. You mentioned lots of change in his life. That change is probably the reason for it. Go slow. Don't push. Celebrate if he does what he should. Don't punish if he doesn't. Without anger or punishment in your voice or actions, have him help clean up the mess, and suggest that doing his poops in the potty would eliminate the need for cleaning up the yucky mess.
What kind of "rewards" did you try?
Some kids are good with a tiny treat (a singe m&m or lifesaver candy) EACH TIME. Some kids (older ones usually) do well with a sticker chart where they get a bigger treat (toy, trip somewhere) after a certain number of successes. Is there something that he Really Really wants (a train set, basketball, nerf gun, lego set) ?? Buy it. Show it to him. Explain that it is for when he no longer needs "pull-ups" and can use the potty full time. (no criticism or judgment - just matter of fact). Keep in mind throughout, that if the same style isn't followed at dad's house on the weekends, it will take longer. If mom and dad are recently separated, he is going through a lot, and is probably just trying to maintain some control of SOMETHING in his life, so don't force or punish.
Best wishes to you all.