My ExHusband had these probs when he was 6-8 yrs old, my MIL had to take him to the ped to 'help' the BM when it got stuck because he would NOT go, he outgrew the prob on his own she never said anything about cleaning up after him tho', I know when I have been constipated for awhile (I have MAJOR tummy probs) when my body finally decides to 'let go' there is no stopping! It may be the same with him, he simply can't control @ age 6 to make it to the BR in time. Make him understand that the MOMENT he feels that urge he MUST go immediatly to the BR, explain that mommy is PG & she gets sick when she has to clean up, I am sure you are frustrated but TRY your best to remain loving & kind while explaining.
I agree with Jo W., try the explaination route about the body & how it works, could be a very good thing, there are sites dedicated to children & books @ the library that you can take out & read with him.
I don't know how involved the appt. with the Gastro. Ped. was but if this continues it wouldn't hurt to check out his system to make sure it's NOT something medically related.
Just FYI, I NEVER did go when I was in school either, not even to pee unless it was a dire emergency! I STILL can't abide public restrooms & only use them when it is absolutely necessary.