Not sure if you've explored this yet, but have you considered eliminating dairy from his diet? My daughter started having "colds" all the time when I quit nursing and started using formula. She finally got a sinus infection at 11 months, and another soon after that. We took her off cow's milk and put her on goat's milk (would have done soy, but there's no fat in soy, and babies need the fat...). She hasn't had hardly a sniffle since. We switched her to soy at age 2.
Also, my son had bad sinus problems last year at this time (actually, beginning in October). After about 5 or 6 antibiotics we had his adenoids removed. He was sick once after that. This year, he has had just 2 antibiotics since September. We have determined that he's allergic to something that happens or begins in October (maybe some sort of mold). Also, any time we give him Benadryl he ends up needing a course of breathing treatments. So, this time around we have not used Benadryl and he has not needed breathing treatments more than once. We keep him on Zyrtec to help control the allergies, and when the congestion starts happening we only give him children's mucinex (you have to find out the right dose from your doctor).
There's a great allergist in Flower Mound that helped -- Dr. Hudelson at Family Allergy and Asthma ###-###-####, and then Dr. Alan Murray is a children's ENT in Coppell -- www.entforchildren.net ###-###-####. They are both great!