E.J.
Hi,
If your son didn't have sinus/allergy issues before the new school, it sounds to me that he may have a couple things going on. For the first 4-6 weeks after starting the new school, he should have experienced the "new friends, new germs" phenomenon, which causes colds and coughs. After that, I would examine the hand-washing practices of the school. So many of the germs are transferred from child to child when frequent hand-washing isn't in effect. If your son didn't appear to have seasonal allergies before he started school, it's unlikely they've developed in the last 6 months. When my kiddos get a cold, I give them cherry "melty-med" Zicam, to ease their symptoms. It works amazingly well.
Good luck!
E.