My daughter was 4+ years old when she finally stopped using her diaper for bowel movements. She was a 'withholder' when confronted with using the toilet. Her pediatrician had her on stool softeners and we also tried laxatives, which eventually gave her bloody stools. What solved my problem was letting her use the diaper for her bm's. Yes, at age 4, that gets really old for us parents, but it was best for her. What I think finally resolved the issue was age, and peer pressure. She just wanted to be like other kids and do what the other kids did. Bottom line is every kid is different and you have to figure out what will work best for yours. Good luck. And Happy New Year.
Be sure to obtain the opinion of his pediatrician re: referral to a specialist, but there's no indication in the description you offered that this behavior represents a medical condition. Do continue with your plan for a schedule of sitting on the potty. At the same time, under the guidance of the pediatrician, a regimen of stool softener (and perhaps, but not likely, mild laxative) can be a great assist in having the child overcome the fear of pain often associated with avoidance of the potty. Overall it sounds like you're doing fine. Consider altering the routine the way I mentioned, and perhaps, down the road a little, dispense with the option of using the pull-up - it only contributes to avoidance of the potty. But you needn't make that change right away.