Just a thought, does he go outside at alll? Is any of the blood work abnormal? Or even on the high or low end of normal? We sometimes see symptoms of a tick-borne infection before the blood work shows definitive signs. So, for example, if we have an ailing dog (usually lethargy and inappetence) and the bloodwork shows low (but still in the normal range) levels of platelets and normal range high or low WBC, chances are what's ailing them is a tick-borne disease. They can be devilishly tricky to detect so we usually just start treating with doxycycline on those occasions and that does the trick.
Also, did the vet check his mouth well? I have little experience with cats, but sometimes I think if their teeth are bad, they'll stop eating?
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