D.D.
In addition to the suggestions below, I offer the idea of having your child perform sinus washes twice a day, along with his teeth-brushing. If he's old enough, get him a neti pot. If not, get a SinuCleanse brand atomizer. Here is the link http://www.sinucleanse.com/product/kidsmist.htm
If the link gets pulled, go to sinucleanse.com
A sinus wash twice per day is an amazing thing to help with allergies and sinus problems.