R.M.
Hey S.,
Often kids do not want to sit on the toilet because it hurts or they are scared they'll fall in.
I suggest getting a sturdy stool and a padded child sized potty seat and have your child try it out. Make sure the potty seat doesn't move, or it will scare him.
Since you know he is aware of his bowel needs, lay down the law, "We poop in the potty, not pull-ups or our underwear." Don't buy pull-ups (for popping) anymore and if he poops in his underwear (and it's not an honest accident), they have him clean himself up.
Remember though, do not punish. If you punish, he'll dig in his heals and it will get worse. When an accident happens, just send him to the bathroom to clean-up as if you sent him in to brush his teeth.
Good Luck
R. Magby