Some children are afraid of losing a piece of themselves. Other children are just afraid of the unknown, or they think it might hurt, or who knows what else. Unfortunately, some children just ARE afraid.
It usually takes several successes in the toilet before an afraid child begins to feel more comfortable with using the toilet to poop. My children were fairly afraid for the first week or two, then moderately to mildly afraid for another 2-4 weeks of daily successes.
Where is your ds pooping currently? If he's too scared to use the toilet, then let him use a pull-up or diaper. You don't want him to withhold his poop or to get in the habit of pooping in underwear. Have ds wear undies most of the day, but when he needs to poop, have him ask for a diaper/pull-up.
If your ds poops in a diaper/pull-up, there are a couple of approaches for weaning him off of them. One approach is to have him stand in the bathroom when pooping (for a week or two). Then have him sit on the toilet while pooping. Gradually cut larger and larger holes in the diaper, so pretty soon ds is pooping through the hole, directly into the toilet.
Another approach is to let ds ask for a diaper/pull-up for a few weeks until he is very comfortable asking. Then select a date to begin poop toilet training. On the designated date, when ds asks for a diaper, take him to the toilet instead. Since he'll have asked for a diaper, you'll know his urge to poop is strong, so take advantage of it. You can sit with ds, acknowledge his fears, let him know that he can do it, but insist he sit and produce on the toilet. If you think ds will try to withhold his poop, you can begin giving him a daily dose of a mild non-stimulant laxative while poop potty training to make it difficult for ds to withhold his poop.
Good luck in the process!
mwheland, Hi and welcome to the the Toilet Teaching board.
I agree with the previous poster. The major fear is that they are losing something of themselves. he only thing I would add to the process to try is that when your DS uses a diaper/pull up to poop empty it into the potty and have him flush, and tell him that is it ok for his poop to go in the potty. I hope that helps. Please keep us updated on your DS progress.