My daughter had this issue. She was potty trained, wearing underwear, put refused to poop in the potty. She was really frightened and upset - I still don't understand why, but she wouldn't do it. She never pooped in her undies, though. She would hold it forever.
We stressed that you go poop in the bathroom. Period. She would go to the bathroom, I'd help her put on a pull-up and she'd stay in the bathroom and do her business. I'd clean her up and then put her back into underwear. She did this for months after she was potty-trained.
Does your son like being in big boy undies? If so, I would gently but firmly stress that if he poops in his undies, he's not ready for big boy undies and he'll have to go back to diapers. But give him the option that if he really needs to go in something, not on the toilet, you will provide pull-ups and he can go into the bathroom for privacy. Also, if he thinks it's yucky to be soiled... make him wait a few minutes before you change him. Not in a mean way, just say "oh, I'll change you just as soon as I finish this" and make him wait a few minutes. The yuckiness might spur him on.
If he will not go potty for the sitter - what is he doing? Going in his undies or in a pull-up? I found pull-ups to be totally counter-productive... a kid knows they've got one on and they will just go in it. I wouldn't use them during the day.
This sounds weird and gross, but I really think some kids just like the way it feels to poop. Especially boys. Have you ever noticed that many boys will get a little erection while they poop? It's not something you discuss much or notice, but most moms of boys I know say the same thing. :) Silly, but true.