B.C.
Perhaps you should have a termite inspection? Carpenter ants can also cause structural damage, and they can sometimes start at roof level if they can drop down out of trees. I'm not really sure what else might be a cause of a structural problem. Perhaps a condensation problem might cause moisture to collect somewhere between the floors? I'd move my son to a different room to be on the safe side. You might have to find a way to get a look at the joists in that part of the house in order to figure out what's going on.