If your dog is biting himself or scratching excessively, or has repeated infections in his ears, eyes, anal glands or on his skin, and loss of fur, I would advise a visit to the vet to discuss having your dog allergy-tested. Once you know what you are up against you can exclude these things from his diet and hid environment to make life easier for him.
Food intolerance may be the underlying cause of many of the health problems in animals. We have found this in Fin who is allergic to many and various stimuli. Adverse reactions to ingredients in the diet may well be one of the most common but least recognised causes of ill-health both in pets and humans. In theory, any dietary ingredient can cause an intolerance. In practice, protein (e.g. beef, lamb) dairy (e.g. milk) or carbohydrate(e.g. wheat, rice) sources are the most likely causes although chemical additives such as food colourings, yeast and preservatives can cause a reaction. All the above apply to Fin, resulting in inflammation and sensitivity of the skin, constant ear and eye infections, and loss of nearly all his fur.