5 yo regressing to daily "accidents"

[bringonthetrumpets]

Ok, at my whit's end with ds#1. He will be 5 in Sept and has regressed to peeing and pooing his pants- daily whilst at preschool. I understand he's going through changes. His normal teacher takes the summer off to visit family abroad, so there's that and now a new baby in the house.... but wtf must he act out in this way!? Ds#2 is 3 and we have absolutely NO problems with him regarding potty stuff nor acting out. We don't know how to tackle this behaviour, we don't know whether to start dishing out punishment because he knows better or to start taking his favorite things away, or ignore it. He just doesn't even seem to care nor he does seem to have an explanation as to why he does it. We are so fed up with this behavior as he's well past the point of not understanding how to use the toilet. HELP!!!