R.,
I have had a similiar "nightmare" with my daughter who is now 7 and is doing great. She is highly sensitive and allergic to things in the enviornment. Allergists never concluded just what caused her "allergy". She was on 3 meds at one time. . . and we tried several before we found what worked "best" and that was pretty poor control. The kid was so snotted up all of the time it was horrible. When she was 4 she developed chronic sinusitis. I took her to 3 ENT's, an allergist, multiple trips to the pediatrician, multiple 2 week courses of antibiotics. . .she even had an exam under anesthesia which turned up only adenoiditis. Of course, they wanted to take her adenoids out, I was opposed. So, her regular Pediatrician had been out of the office battleing cancer and one day we saw him. He put her on antibiotics for 1 week beyond when her symptoms ceased. It took 7 weeks. No more infections no more adenoiditis.
I also found a product I love and swear by at Whole Foods, but I have seen it in phamacies, called the AYR bottle. It's bascially a saline sinus wash. We start it as soon as we see any nasal congestion and it controls her better than the 3 meds ever did. So she is off meds completely. All 3 of my girls 2, 5, and 7 have AYR bottles. It will take a little getting use to for him, but will be well worth it.