Well it could be ear mites, an infection or a food allergy, among other things. If you haven't had him checked for mites, that's a good place to start. If he doesn't have them then I'd bring him to the vet to see if it's an infection. You may need an ear cleaning solution along with an antibiotic to help clear it up.
Also, I'm assuming that your cat is young and this would be rare in a young cat but tumors inside the ears can also cause excessive waxy build up.
Also, I'm assuming that your cat is young and this would be rare in a young cat but tumors inside the ears can also cause excessive waxy build up.