M.C.
first off congrats! boys are harder to train so getting a 2 1/2 y old this far is major!
this is a typical problem. I think it has to do with that they are afraid of the poop falling. When they have a diaper on they feel the diaper on their bottoms and then when they are sitting down and poop in it they can feel the poop. does that make sense?
One key thing that starts happening is that they will actually get quiet and or go hide in a corner so you can't see them pooping their pants.
Some things that helped us
- a small training potty with a squishy seat.
- whenever they pooped their pants (at home) they'd get showers.
if they'd say, I don't want a shower, well then stop pooping your pants.