For best results I would go to a veterinary dermatologist, especially if this is becoming a chronic condition and has not responded to treatment by the regular vet. The veterinary dermatologist can do skin testing to identify the environmental allergens responsible then they will discuss treatment options.
If you suspect food sensitivities then the only way to rule out is to have the vet prescribe hydrolyzed food, difficult to follow, commercial brands do not compare.
There is no cure for allergies but there is effective treatment and management.
https://www.acvs.org/small-animal/otitis-externa excerpts below, click on link for full article
Otitis externa is an inflammation of the ear canal. Because dogs’ ear canals are L-shaped (Figure 1), fluid does not drain easily from canal openings. Additionally, the lining of the ear can become inflamed and thickened, blocking air and fluid flow in and out of the canal. Animals with otitis externa can also develop otitis media (middle ear inflammation). Similar to the problem seen in children (especially after airplane flights), fluid can build up behind the ear drum, causing pressure and pain. Otitis externa and media are common conditions in dogs, particularly in specific breeds such as the Cocker spaniel and German shepherd.
In puppies and kittens, otitis externa is often caused by ear mites. These tiny parasites cause terrible itching and a thick brown discharge. In adult dogs, the most common underlying cause is allergies- sensitivity to something in the environment or to food. In older animals, tumors can cause blockage of the ear canal and secondary infection. Other predisposing causes may include foreign bodies (such as grass seeds), or small ear canals (often seen in Shar peis) or long floppy ear flaps (for example, Basset hounds) that prevent air flow. Hormonal problems, such as poor thyroid function, or other underlying skin disorders may also be present.
