R.D.
If they are 'always updating,' then there is the potential for a lot of dust in the vent system, especially in an old house. He could be breathing that - or mold, which can hide most anywhere, even inside walls.
He might also be allergic to the bedding. Does he get sick just after a long day visit? Like from 8 am - 8 pm? Or is it just if he stays over?