My 4yo was doing the same thing. Hadn't been in diapers for well over a year, even at night. But still not taking himself to the pot to poop. I was at my wir's end- frustrated, angry, worried, the whole bit. I'm sure you can relate.
Finally I came up with the idea to write a contract. So I told him that I know he doesn't like pooping in his pants, and having to get cleaned up. So he needed to sit on the pot everyday. I asked him when ge would like to do that. I made some suggestions (9:00, 9:30) and he chose after breakfast (which is perfect if you ask me- he's just eaten, we haven't left the house yet, he's not in the middle of anything). I asked him how long he was going to sit (5 mins, 10?)- he chose 4 minutes. He always sits longer, so that's not been an issue. I wrote it up and we both signed it. I even let him decide where to hang it (he chose the fridge). He had a lot of ownership over the contract, which helps I think.
So that was several months ago. He's grumbled about sitting, but I kind of ignore it, or I take him over to the fridge to remind him that HE signed it and it's non negotiable. Do you know we've gone from several poopy messes per day to a few a month? He's pooped nearly everytime. If he does't he'll have an accident that or the next day. But I'm calling this a success in that he's actually pooping in the potty.
Now, I'm not real sure how to get him to recognize on his own that he needs to go, but I'm holding out hope that he'll get it eventually.
Best of luck to you!
Oh, I should add that he seems to be comforted by "potty time". He's happier not having all those accidents.
~helen~ mama to 5 yo twins jonas and micah and my 2 yo baby boy eli