Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 16 years of experience. I look forward to working with you.
Great job so far on what can be done over-the-counter.
There are many possible causes for intense itching such as this. Our top culprit is an underying allergy (such as flea allergy, atopic dermatitis which is environmental allergy or food allergy) which led to a severe secondary skin infection of bacteria, yeast or even mites. Sometimes the mites can be a primary infection. This is something that I am afraid at a high level of itching to the point he will chew himself raw will need prescription medication (to treat the active infection and clear the itching), and that will require a visit to see his veterinarian as soon as you can arrange this.
In the meantime, I do need to confirm you are using effective monthly flea control (as a note Frontline over the count does not work well these days, and all of its knock-off fipronil-containing derivatives are not good choices). The only over-the-counter option I might trust is Advantage (or Advantix). There are a lot of products you can get from your vet that work well.
Assuming you are giving Duke effective monthly flea control and we can rule out fleas, then the only things you can do over-the-counter really are what you have already done. Make sure the Benadryl is enough (1mg per pound every 8 to 12 hours). For shampoo, I recommend use something that will actually treat. If you have a medicated shampoo you got from your vet then this is a good start, but otherwise, I recommend try Selsun Blue (original). You are welcome to do another bath if you would like. For sprays over-the-counter the only one that might provide some temporary relief contains pramoxine, although it doesn't work well. Longer-term a good omega-3 fatty acid supplement (e.g., Free-Form for dogs) can help reduce itching but will not clear up what is going on right now.
I recommend you get him in to be seen tomorrow unless you feel he is in some way declining tonight and then have him seen on emergency.
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