You might want to talk to an allergist (who is probably also trained in immunology) about why your son has so many sinus infections and what can be done to prevent them. Allergy treatment might help (such as Flonase or Nasalcrom). Avoiding an offending food might also help (many who have frequent sinus infections are allergic to milk, for instance). Avoiding irritants can help--exposure to cigarette smoke can cause many sinus infections.

Another gentle thing to consider is Xylitol, a nonsugar sweetener that can cut the number of sinus infections. It comes in sugarless gums and mints, and as a powder.