At this dog's age I would doubt that he has mites. I would more likely suspect a bacterial or yeast infection.
You should see your vet so they can do an ear cytology (take a small sample of the discharge and put it on a slide, apply some stain and look at it under the microscope), to determine what exactly is the cause.
They will then prescribe the proper medication to treat the infection.
Ear infections can be caused from a number of things including allergies, thyroid disease and autoimmune diseases.
You should talk to your vet about the cause and what you can do to prevent them.
I hope this helps, let me know if you have further questions.
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My answer should not take the place of your Vet's recommendation.
Vicki CVT
