A.P.
Potty training is harder for some kids. A couple thoughts... first, my last two boys (I have 4 kids) potty trained way-late. The 3rd child went on the potty 2 days before his 4th birthday, and the 4th child 2 weeks before his 4th birthday. Here's the thing. Healthy children are all potty-trained before kindergarten so we don't need to worry. :) Trust that it will happen.
You didn't mention if he was having accidents with pee or poop. If it's poop, you might give him the option of going on the potty or asking for a diaper. With my first 2 kids, I gave them the option of going on the potty or asking for a diaper, and they were "tinkle-trained" for a few months before they were willing to poop. No big deal because it saved me from having to clean up accidents, and I was able to be a better mom.
Hang in there! He will potty train.
A.