M.B.
L.,
He may be withholding because the splash as the poop hits the water and hits his butt is scary. That was the major issue I had when I was potty training my boy.
At 2 it will be a little harder for him to articulate, but if he's super verbal you may want to try and talk with him about it.
When I learned that we had "monsters" in our toilet that were trying to get my son/his poop we changed the way he went poop on the toilet. I placed an IKEA little step next to the toilet that he could use to climb up on the seat to squat over the toilet. Once I was sure he could get up safely, and he knew how to position himself I'd let him do his business. We never had another poop accident in his pants.
Hope this helps,
Melissa