Hi There,
My daughter is almost 3 and 1/2 and just started consistantly pooping in the toilet a month ago. She has been pee trained and in underwear with NO pee accidents since last summer!! The poop thing was a control thing, and although she knew exactly what to do, she would not. The more we tried to get her to do it... the more she would control it. Literally, she would not go and get constipated. So, our pediatrician said to just wait until she was ready. We just made the rule that she had to ask for a diaper and go in the bathroom... that is where we poop. She would always ask for a diaper, and go in the bathroom (but mostly poops early in the morning when she still has her night time diaper on anyway, which I think made it too easy for her). So, we finally decided to put a little potty seat in her bedroom with towels under it and said, here is a potty if you have to do you poop in the morning, you can do it by yourself. And sure enough, a couple of days later she yelled to us "hey, I just did my poop in the potty and I need help wiping!". Now we leave some wet wipes there, and she does that herself too (we check of course)... and she uses toiled in the bathroom if she needs to go during the day. I want to wean her from pooping in the potty in her room, but atleast it is in the potty!! So, my advice is, don't push it, just let it go for a little while. Maybe have a designated poop potty and say, "when you are ready, here is a potty to do your poop in"... and just wait a while.... Another thing to do is get rid of pullups alltogether, they act just like a diaper, he might be more reluctant to soil his underwear... then make a rule that if he needs to poop, he needs to ask for a diaper and do it in the bathroom, or use the potty. He may decide that the process of putting a diaper on and getting changed is just not worth it, and go on the potty instead.
Good Luck!