Veterinary questions? Ask a Vet for Answers ASAP
Without further information it is very difficult for me to provide the best advice but I shall certainly try.
Please reply when you can and we can continue the conversation.
I would consider fleas, mites, a bacterial infection, stress or an allergy.
Some cats are more sensitive than others so the other cat being OK doesn't rule anything out.
Ensure the parasite prevention is effective against both fleas and mites e.g. Advocate.
As he sounds quite itchy, a vet visit is sensible. They can examine the skin, prescribe anti itch medicine to allow the skin to settle and perform any diagnostic tests needed such as a skin scrape or swab.
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