How was his transition into JK?

Is he stressed about school or anything at home?

Is he too scared/shy/worried to ask to go to the bathroom. What is the set up/system at school?

Something weird is going on. It's been a few months so it isn't merely a regression.

It may have started as a 'normal' regression and then he liked the responses he got so he's stuck with it, becoming a habit. But the fact it goes on is all settings seems suspicious. I can see him liking the attention/reaction he gets but ALL settings likely don't respond in the same way so I would think it would be limited to specific settings.

If you don't see any signs of anything going on (developmentally, medically) then try and put ALL negative impact on him. He changes his clothes, he gets new clothes, he washed the floor, cleans the poop off himself with no help. It will suck for you but just section him off somewhere that the impact is less on you (he has to stay on the tile bathroom floor until clean). See if that gives him less incentive. I would just caution that this is NOT appropriate for a child that has underlying disorders as it is punishing them for something that truly is out of their control. It's a perfectly fine consequence for a child that is being lazy or is just trying to be a 'pain' for kicks though!