T.S.
I wouldn't punish him when he does it. It may not be a control issue but rather a stress problem. Try a positive approach. Praise him for being your big boy and remind him how proud you are that he goes pee and poo in the potty. Maybe with a bit of positive reinforcement and over time getting settled again things will get back to normal. If you want to save your time and hassle you could put a pull up on him, however I don't know if that would be a good idea....try it and see if he regresses more.....
Good luck, congrats on the new baby and good luck with your move.