A diet change might help bandaid the symptom for a while, but we really need to get to the root cause as to why his sinuses keep singaling that there is a problem.
As people age, their bodies get "tired" of fighting common everyday contaminates such as air fresheners, cleaning products, etc. If the removal of dust helped, then he might just be sensitive to dust, mold, pollen, etc. due to his age.
People do not realize just how many different chemicals we are exposed to each and every day. A liver cleanse might help his liver be able to remove toxins faster which in turn might lower the irritation to his sinuses.