K.L.
It seems to me that kids don't potty train when we think they should for one or two reasons. One, they are using it as a control issue. They may feel powerless about their world in general so controlling where they go potty is all they feel they have left. Two, is he afraid of it somehow? Is he constipated or has been constipated or had a bladder infection? I understand that kids who associate pain with going to potty often have trouble potty training. I have also read that some kids see their poop, in particular, as an extension of their own bodies. So when you flush it, it feels to them like you are flushing away part of their bodies. I see from my friends and family that boys do take longer in general than girls do to finally complete potty training. I have one friend who has a very bright young man who took till 4 plus to finally go poop in the potty. It may help to talk with his pediatrician as well.
Best wishes, breath deep.