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What it sounds like you are describing as occurring with your dog is problem with underlying allergies. These can make a dog's skin itch and there may or may not be other skin lesions visible. Itching problems in pets can be difficult to diagnose and the history and response to medications tried are a very important part of figuring out the cause.
As far as the cause of the allergy, dogs can be allergic to three things: 1) fleas (different than a dog with fleas, these dogs are sensitive to even one flea bite), 2) Food (they develop antigens to some component in their food), or 3) inhalants like dust, pollen, molds, etc. On top of allergies, this affected skin can then be invaded by bacteria or yeast which can add to the itch, even if the original allergy has now subsided. When these bacteria or yeast live there for a long time, the skin can develop secondary lesions like hot spots and even become blackened and thickened.
Antihistamines are sometimes tried for control of the itch in allergies. It works for some pets, but not for all. It is usually not very effective when there is a bacterial or yeast infection present. Another thing that will sometimes help in mild cases is fish oil or omega 3 supplements. A test called a skin cytology can be done to look for secondary invaders like bacteria or yeast. There are other things to look for as well that can make a dog itch and cause skin changes like mange mites, ringworm. Sometimes we need to use corticosteroids to control the itch, but only after we make sure we have treated the secondary issues if they are involved as well. Even if you are not seeing fleas, he should be on a flea prevention product if he is an itchy dog.
It is very important to work with someone who understand allergies and can look at his history and seasonality of itchiness, results of dermatologic exam and response to medications and put it all together. I would schedule your dog for a physical exam including a thorough dermatologic evaluation with your veterinarian if he hasn't been seen yet. They will be able to best determine the appropriate medication to prescribe for him based on his current condition. Allergic dogs can be very difficult to manage, but with close communication between you and your veterinarian, you should be able to help him. Don't be afraid to get a second opinion or ask your vet for a referral to a veterinary dermatologist if you feel you are not making progress. In the meantime, he may also get some relief from cool water baths and Aveeno oatmeal (human product) shampoo.
I hope you find this helpful and please let me know if you have any other questions.