What you describe sounds like environmental allergies. If I were you, I would consult a board certified veterinary internal medicine specialist or dermatologist. Diet/food may not have much to do with his symptoms.
At age 16, I would let the dog (if it was mine) eat whatever he wants, in moderation.
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I assume your pet has had a senior workup, if not, that might be a good place to start. The veterinarian may be able to prescribe medications (depending on the results of the exam) that will keep your dog comfortable.
