Hi,
I, too, like to avoid prescription or OTC medications whenever possible. Last spring, my then five year old son had very bad seasonal or environmental allergies. I began giving him the following nutritional supplment that has quercetin and nettles in it.:
http://www.naturessunshine.com/us/product/histablock-90-c...
I broke open the capsule and sprinkled it in a drink or in his cereal, applesauce, etc. It worked. I've even used it myself and recommended it to my Mom. It has worked for all of us.
Also, I'm reading a really interesting book right now called "Let's play doctor!" by Dr. Joel Wallach and I also listen to Pharmacist Ben Fuchs on the radio in the morning. Both of them talk about how sinus problems are an immune system response, and they also talk about how a large portion of your immune system is in your digestive tract. So, your son might also be suffering from food allergies. You can test this at home by using a Pulse test. When he gets up in the morning, you take his pulse. Then, feed him only the food that you suspect may be an allergen. Then, take his pulse again 15, 30 and 60 minutes later. If it goes up by 10 beats per minute, he is allergic and should avoid or rotate that food. Along those same lines, if he isn't fully digesting his food, that could also cause problems for his immune system. I also give my son digestive enzymes. Here they are:
http://www.naturessunshine.com/us/product/proactazyme-100...
That may help also. Good luck!
C.