Alisha, Hi and welcome to the Toilet Teaching board.
For your DS it is normal for a child o take longer to learn to poop in the potty. You many want to try having your DS ask for a diaper when he needs to poop, but have him go into the bathroom. When he is done have him help clean up. Do that for around a week. Then continue to have him ask for a diaper, but this time he needs to sit on the potty. Then after a week start cutting a hole in the middle/back so when he sits on the potty most of the poop will fall into the potty. After another week try removing the diaper. If your DS still isn't comfortable, try putting a belt or tie something around his waist, this will help the muscle relax.
Instead of the pull-ups,go to cloth training pants on both ,I think you will find,neither will like the feeling of being wet,as for your son,pooping is the one area of training that they have almost complete control over,but you can try Milk of magnesia to loosen his bowels so he cannot just hold it for long.
If you worry about messes and pee on the furniture or so,use plastic panties over the trainers,but stick with the cloth,even when going out.
you might also try rewarding them for potty action,say one M&M for