I've had this issue twice. I would not push the issue. I would wait. My son (now 8) sounds like yours. It was closer to 4 that he FINALLY sat on the potty for a BM. And that was because we were out at a store and I did not have a diaper on me (on purpose, because I knew after 3.5 he could handle it) and he had to go. So I said, "look, here is a toilet, it's private and I'll be with you. You are safe." In desperation, he went. A trick I used that worked wonders was taking a picture of the poopie (I always carry my digital camera with me, now phones take pictures) to show Daddy. He loved that idea. I still have a digital photo selection of a few of those poopies.
My daughter is 3 yrs, 9 months and she JUST started using the potty to have bowel movements! Same issue... fear. For months, she would ask for a diaper and then run and hide (for privacy, she didn't want anyone looking at her) behind the couch to poop. I never pushed the issue, knowing things WOULD change one day. Two weeks ago, I ran out of diapers (at home) and I told her if she did it in the potty, I could take a picture of it and show her Daddy. So she said, "OK, I'll sit on the big toilet." It worked! She has been going on the toilet ever since and if she asks for a picture, I'll take it, no arguments. She doesn't ask much. But both were very proud of their poopie photo and wanted to show it to their father.
I really wouldn't push it right now. Just give it time. I'd be stubborn too if I thought I would fall in, since I was so little and that toilet is so big, ya know?