M.M.
All I can say is... my older daughter was exactly the same. She was trained for months, but would ask for or go grab a pull-up on her own to poop. The first time she did it in the potty - this is what I heard "it didn't splash me. it didn't hurt. it wasn't too loud" ~ she had all those fears. I laughed so hard (not in front of her) but that is why she had so much trouble. Maybe talk to him about it - maybe he has some fears - irrational or not.
Good luck!