Hello and Welcome!
My DD 7 (Highly gifted) potty trained at 3 and we are in the process of potty training my younger DD3.6(untested). My children have generally potty trained later than most of our family/friends kids. Like your GS, DD3 has taught herself to write the alphabet and loves puzzles. She doesn't have a problem urinating in the potty but we've noticed that she is not comfortable having a bowel movement in the potty. I've put her in pull ups the past couple of days because we believe she's been holding her bowel movements. She is also in pull ups at night.
I second aeh's question of whether your GS is holding his movements in general or specifically because he doesn't like the potty. If he's holding his movements in general, I would speak to your doctor. If it is potty related I would continue to encourage him to urinate in the potty and keep pull ups on for bowel movements.
Anecdotally, I have a friend whose little boy didn't go to the potty for bowel movements until just before he started kindergarten, and in the end everything worked out fine.
Is your GS able to articulate why he doesn't like having a bowel movement? Is he experiencing pain or is he uncomfortable? We are pretty open with our kids in terms of talking about bathroom issues and my DD7 is very comfortable talking to us if she's having difficulty. If your GS is not comfortable I would follow his lead. If he's in pain or generally avoiding bowel movements I would speak to his doctor.
