Like some others, I wondered if this might not be an issue of underwear fit at all but a medical or toileting issue. He may as others noted have pinworms, but it may not be even that serious -- it's possible (sorry, have to say it) that he doesn't wipe poop sufficiently and might have dried stuck-on poop that makes his backside itch, feel stiff, etc -- the stiffness on the skin would feel a lot like a wedgie, and also if there's poop on his skin, his underwear is going to stick to it and dry there, hence the underwear up the crack. Even a small amount dried in a spot you can't see just with a glance at the backside could be an issue causing discomfort. I'd get him checked by the doctor for pinworms or other poop issues and I'd ensure he's using flushable wet wipes for poops at home. Not sure what to do about school, though. Maybe have the teacher let him keep some wet wipes in his school bag, but will he remember to use them, or fear others will ask about them?....