A.H.
That is also what happened to me. I just put him back in diapers and hoped that when summer came he would get to hot and not want the diapers any more. Well that is what happened and he just told me he did not want them any more and I said to him that he had to in the potty or I would put them back on and he did. Sounds easy but it was not. It is not worth fighting with them about something they are just not going to do. Plus I think he was jealous of the baby and that was his way of getting back.You are giving him attention. They don't care what kind of attention they are getting as long as they are getting it.
Good luck
A.