Both my sons, and even I in my 50's have chronic problems. All I can say is after years of different doctors, the one who helped us the best was the head of peds ENT at University Hospital in Cleveland. As a nurse, I would suggest you seek out the largest teaching hospital you can find in a large city and try to see the top guy.
I don't think it is good that your son has had this reduction in hearing for so long. He must be missing a lot of his education this way.
If you are giving him Claritin plain, it is an antihistamine which will dry him up but not help the mucus drain. Sudafed is a decongestant and will reduce the swelling in the inner ear and allow the fluid to drain into the eustacean tubes. Mucinex is also helpful by liquifying the mucus so it can drain.
If he is otherwise healthy, perhaps giving him a two week trial of sudafed daily, as the packaging recommends for dosing, to see if he gets some results. This would be helpful to report to your doctors as well.
Some of us are just born with narrow ear canals and narrow eustacean tubes. That is why I still have issues at my age.
Good luck. Seek out the very best doctor you can find and see what they say.