Hi A..
I agree with previous posters: in my experience as a nanny and preschool teacher, Pull Ups is "magic underwear" for little ones and very confusing.
I'd use underpants as much as possible, and offer a diaper for outings. Children can differentiate between diapers and underpants, but PullUps are too ambiguous and I've seen them prolong toilet learning.
Younger children may often wait to use poop when they are wearing a diaper. This is really, really common. There are plenty of youngsters who don't get comfortable with pooping on the toilet until they hit their 3's or so. Please don't worry-- know that he *will* eventually get it. Kids sometimes have to feel proficient with using the toilet for urinating before they can become comfortable enough for the sphincter muscles to relax, so be sure to keep toileting times low-stress and just let him go at his own pace.
Good luck and I wish you plenty of patience. This is one of those learning times that require plenty of it!