L.U.
He is not having accidents. He is upset that there is a new baby in the house and is letting you all know it!
So, you say that babies make messes in their diapers and clothes. Since he is still pooping and peeing in his clothing that he is going to need to go back into diapers. (said in a sad voice) Then you say something along the lines of "You are a big boy and we were SO PROUD of you going potty in the potty. But, you seem to be forgetting how to do that, so we are going to put you back in diapers until you remember. When you want to go potty in the potty you let us know" and then you wait. When you go on vacation you should bring diapers for both.
This takes the stress off the mother (cus it is stressful cleaning up pee and poop clothes all the time!!) and stress off the boy. Now he gets to decide when he is going to be a big boy.
L.