J.B.
Well, I don't subscribe to the theory of putting him back in pull ups...I feel that will only make trying to re-train him harder and you're sending him mixed messages. Since he was trained for three months prior to the baby, give it some time. Like others have said, it's a big adjustment for him. One thing you can do to minimize the damage is buy rubber undies that go over his big boy underwear. Then come up with a reward system. First reward him perhaps with a sticker every time he goes in the potty and two every time he poops in the potty. That way he's getting positive attention for going potty. Tell him "after ___ stickers, we'll take you to the dollar store and you can pick out a toy". What I did was fill a chest with dollar store toys and let them pick one out of the chest after so many stickers. It worked like a charm! Good luck and hang in there! :)