My son started being willing to sit long enough on the potty to poop the week of his 3rd bday. He was peeing on potty for about 6 months before that, but would hold it until he got a diaper (nighttime) for poop.
We went and bought him some books from the bargain tables at Barnes and Noble of his choice, that he could only read for poop on the potty. Initially it took about 30-45 minutes to get him to poop - sometimes I read the same book 9 times! After a few successes, he realized that it was much less "sticky" to poop in the potty than in his diaper/big boy underwear. We did let him see both Mama and Daddy go poop in the potty and flush it down. We also know that he needs to have the step stool under his feet for him to "push" to get the poop out. Now if it isn't there, he sometimes asks for it. By the way, my son turned 3 on 4-11-09 and has been pooping in the potty (no accidents to date) for about 6 weeks now.
We also gave him an increase in raisins and apple juice to try to soften things up a bit when we were having difficulties. Now we have no problems at all.
Your son will poop in the potty too! Hang in there, see if the other sons will demonstrate or read books!
C.