The antibiotic injection used was Convenia. It's usually really good for skin problems.
It's difficult to say without seeing him in person what this might be, but I would suspect something along the lines of a deep pyoderma or dermatitis. These types of afflictions are fairly common in younger cats. In some cases, even ringworm can cause itchy, raised areas that have patches of hair that will come out easily when irritated. We usually see the pyoderma and dermatitis happen around the neck, chest and sometimes even into the lower abdomen, inner thighs and around the genitals. For a young cat who has not responded well to treatment thus far, we usually recommend sedating the pet, clipping the area, taking samples for a culture and sensitivity, as well as biopsies. A fungal culture would be useful, as well.
The clipping of the area allows us to easily topically treat the area and to see what's below the fur. In many cases, the lesions extend 2-3 inches past where the area appears to be irritated or you're feeling the bumps. Depending on what is seen, topical treatment may be recommended. The culture and sensitivity samples will help determine what antibiotic will be a good choice (convenia is a good option, but sometimes what we're treating is not susceptible to our antibiotic which, in such as case, wouldn't cause a response for the pet). Biopsies can be useful in determining if something is wrong with the skin. These are typically done in pets that are older, but in younger pets who have an odd presentation they're a good choice as well. These tests can be expensive, but give veterinarians the best shot at not only a useful diagnosis but also a useful treatment. Depending on the culture results and what the skin looks like, your vet may also be able to use the culture results to have a custom-made cream made to apply to the skin to give direct results. Another useful approach would be a tape test, as this would be useful in checking for bacterium, the presence of yeast, external parasites, etc.
If you're unable to yield results with such an approach, a referral to a veterinary dermatologist would be the most useful. They are skilled in working with conditions that might stump veterinarians. You may well be able to save a few bucks by having the tests mentioned above performed with your regular vet instead of a specialist, too.
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