Boys always take longer and are older. I know several that were 4y+ before they got it. For my brother, (4y+) it took the promise of a special half his size truck before he got it. Being overly intelligent does play a role in that they've learned how to play the parents, but doesn't really come into play here.
First I would have him checked to make sure that there isn't a medical issue. Bladder is too small; UTI; etc.
For my son, it was that he didn't like going into the bathroom where it was dark. So he'd rather mess his pants then go in there. For that I placed the little potty into the open laundry room. No doors or dark spaces.
Secondly, when he would mess, I would rush to clean up the mess on the floor or furniture first. He would stand there getting upset because he was wet and uncomfortable. I didn't make him wait long, but like 5m until I finished cleaning up the location. Then when it was his turn, we'd have to go ALL the way up to the 2nd floor bathroom, take a shower to clean up, and put on fresh clothes. He got tired of taking showers. 'well, each time you mess your pants, you get a shower'.... it was only a few days before we were accident free during the day.
Change his clothes ONLY in the bathroom. Not his room. Not the living room. ONLY in the bathroom.
Hugs
M.