Is he still eating wheat?
That's a long time to go w/o a bowel movement.
Another thought ... you're going to think this is weird, but are you sure he isn't going somewhere else and putting it somewhere? My son hated to wipe himself and was afraid of his feces and I cannot tell you how many times we yelled at the dog for pooping in the house. We couldn't figure out how it had gotten on the walls, either. Sigh.
I don't know why people think these kids aren't smart because they are. It's we parents who just don't see them flying under the radar.
Anyway, just a thought ...