Have they tried a steroidal nasal spray?
What about saltwater nasal rinses?
When I was dealing with sinusitis from allergies, I was prescribed the steroid spray. I have had better results with a salt water nasal rinse though. You just need to get a bottle or pot like in the pic and then mix a solution of about 1.5 cups of distilled water to 1 teaspoon of salt(add a couple of pinches of baking soda to the water to prevent stinging). Then squirt the water solution up his nose with him bending over a sink the solution should come out either the other nostril or his mouth.No inhaling.(I have never used the little pot method so I am not sure how to do it but it usually comes with instructions).
Basically he has an infection because the mucus in his nose is too thick to do its usual thing which is just flow down his throat and it stays there, then he gets an infection. He needs to flush it out to make it more fluid. Salt water is also good for dealing with infections.
Try to google for other home remedies for sinusitis.
Both worked but I prefer the saltwater rinse better, its also cheaper.
*edit I also gargled with a salt water solution too.
**edit here is a link http://allergies.about.com/od/noseandsinusallergies/a/nasalsalineirr.htm