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I am sorry to hear that your cat is not feeling well. I hope I can help. There are multiple reasons that we may see weight loss in the face of a good appetite. These include cancer (but he is young for this) viral or bacterial infection, the constant need to fight or breed, (if he is still intact), poor GI absorption of nutrients, inadequate calorie intake (he is eating, just not enough to meet his needs for some reason). His eyes look sunken because they are. There are normally fat pads around and behind the eye and his need for calories has caused him to breakdown this fat for energy thus giving him a sunken look. Additionally, dehydration will contribute to the sunken look because dried out tissues have less substance than well hydrated tissues.
The best way to know what is going on with Kevin is to have him seen by a veterinarian that can perform a physical exam and possibly some blood tests or imaging to look for the above causes and more. If you do not want to or cannot spend a money on diagnostics, you cans talk to your vet about empiric treatment (this is essentially treating the treatable things or see how he does on an appetite stimulant or course of antibiotics to cover for infection.
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