Hi A.,
I also had this problem with my son. He was almost 4 when he finally decided he was ready to be a "big boy". For him, it seemed to be a competition-type thing with his "best friend" in daycare. Plus, he seemed to inherit stubbornness genes from both sides of the family, and seemed to get a double dose! Fortunately, he also got an extra helping of goofy genes, so they seem to balance each other out.So here's what we did. We finally decided that since pushing him wasn't working, we put the onus on him to make the CHOICE since there's no "making" this kid do anything he's not ready to do. He just digs his heels in deeper and becomes more determined if we attempt to force him into something he doesn't want to do. Not that I don't try, mind you. I've got the stubborn gene too.
We made a clear distinction between being a "big boy" and being a "baby" explaining that along with making the choice to be a big boy (which was up to him) came some pretty cool perks. We explained that babies aren't able to play with certain types of toys, participate in certain types of activities, etc. Conversely, big boys get some extra privileges that babies do not get such as helping dad in the garage, helping both mom and dad to cook, eating at the table without a baby seat, etc. We tried not to pressure him too much, but attempted to instill in him a desire to make this grand leap into big boyhood. We got him a really cool toy but told him that since it was a big boy toy, he had to earn it by becoming a big boy. We also got him some really cool big boy underwear with his favorite characters and asked him daily if he was ready to make the choice, showing him his cool new underwear. If he chose not to, that was OK, no big deal. We just asked again the next day. It took a short while for him to make the choice, but once he realized the benefits of doing so, he was done with his Pullups and hasn't looked back. By the way, Grandma was actually the one who got him to wear the underwear in the first place. If you've got one of those handy, they're a tremendous resource!
Good luck and hang in there. You're not alone and there's nothing wrong with you or your kid. He'll get it. I understand the frustration entirely, but you're almost there.
~ S. M.