I would ask your veterinarian for a dermatologist referral. Chronic ear infections are usually the result of allergies and they have very specialized training in this. I have seen many dogs come back with some different types of treatments for the ears that work after seeing a dermatologist. Expect that they may want you to change his diet, because many times the allergies is related to a component in the dogs food.
If you want to try this without seeing a dermatologist then consider allergies to be the biggest factor. Try a food trial using IVD duck and potato or venison and potato. With a food trial the dog cannot have any other foods or treats by mouth. It will take 8-12 weeks to see results. Start your dog on antihistamines. You can try benedryl at a dose of 1mg per pound three times daily. It would be best to get this most recent ear issue cleared with either antibiotics or antifungals first depending on which type of infection he has bacterial or yeast. His ears should be flushed daily while he is having a problem either using a cleanser that the veterinarian has prescribed or mixing vinegar and water. Fill the ear canal completely, massage at the base, let him shake his head, then wipe away the excess with a soft cloth. This can be repeated. There are a number of antihistamines on the market for dogs with allergies and this usually is a trial and error process. Zyrtec and Claritin are now recently being used with some amazing results.