A.W.
This is a cry for attention.
Absolutly DO NOT put him back in diapers. A new baby is a tramatic event in a childs life. While understanding that and trying to be a little more attentive to him, you need to let him know that the behavior is NOT okay. I think a combo of time-out and making him clean up the mess EVERY time will nip the problem in the bud quickly.
It is also EXTREEMLY important to communicate with him. Let him know you understand the change is hard on him but how exciting it is at the same time. There are childrens books that directly relate to this issue that would be a good conversation starter at bed time or story time.
If possible give him his own special day or afternoon EVERY WEEK where it's just him and mommy or just him and daddy or just him and mommy and daddy. Just doing the weekly shopping with only him, and allowing him to be a big boy and find items and put them in the cart will work wonders. But remember: no baby.