It could be enviormental allergies even though he doesn't go outside pollens get in side. As a allergy sufferer I know it's impossible to completely keep it out. I moved to Montana and mine and my sons have disappered. In Washington we were drugged out 8 months out of the year. When we visit the poor kid is only there 2 days and on full meds again,I seem to be able to handle a week before needing any thing.
Hope it is just allergies.