J.K.
Hi B.,
My daughter did this when she was potty training too. I swear, she didn't know how to poop sitting down - only standing. She would ask for a Pull up to poop, and then she would go. I would ask her if she wanted to try, and finally one day she went, and after that, no more pull up. It's a big adjustment trying to go from a diaper/pull up to the potty, but eventually he will get there. My only suggestion is to keep trying to get him on the potty and hope that he goes. Once he feels the difference, I am sure it won't be an issue any more.
BTW - Most of my friends kids went through this, so it isn't at all unusual!
Good luck!
J.