He could have picked up ear mites from other dogs. Sometimes you can't see them until they really start multiplying. They look like flaky brown exudate down very deep in the ear canal and then usually build up to where you can see them. If he starts shaking his head, that is another sign. You can easily treat this by placing a Revolution application on him which is flea prevention and also treats ear mites. You have to be current on heartworm prevention to use this prodoct, though, as it is also a heartworm prevention.