Dear K.,
I watched Dr. Oz on Oprah a couple of weeks ago, and he said that the human bowel system cannot be forced, it operates on its own time, and that is known in the medical world as.... if I can remember the proper words, something that means that the human has to feel that he or she is in a safe place, and will not be bothered or harmed while the bowel is working. Something like a safe harbor bowel, because in the extremely olden days they had to do it outside where a wild animal could creep up on them, so that the body wouldn't allow the poop to come out if it were not in a safe environment.
All of this means that the doctor was right especially in his closing sentence. Find the right time. You might give him a sweet dried prune just before bedtime, and he might be ready to go to the bathroom soon after getting up. Give him water right away and a sweet prune just before bed. They don't taste as badly as they used to when we were children. Of course if it is the right season you can give him a plum to eat. I used to tell the kids that it was candy fruit - without the pit.
He is in very exciting and interesting part of his own individual life - imagine going into the 3rd grade and being in a school with up to the 5th grade, by the way, younger children are truly scared of the older ones, and rightfully so. I used to be an elementary school teacher and recess time is scary time for some younger kids. "Of course not in your school, people will say." Just be aware - that is not a good time or a safe place for his bowels to move. So that is why he won't go at recess time. We are complicated people aren't we? Just keep on trying and remember 3rd grade is the best, the kids tell great jokes and have a happy year - preparing, I suppose for the 4th grade that, I hear, is where their behavior (not your son's, of course) gets 'bad'. Stay close to him and to them. Sincerely, C. N.