Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I too am concerned that Clyde is getting very thin even with a good appetite.
When we see weight loss with an apparently normal or increased appetite we need to worry about disease processes that either don't allow him to digest and/or absorb the food he is taking (such as diabetes, liver disease, primary intestinal or pancreatic disease including cancers) or those that lead to a higher metabolic rate such as hyperthyroidism or cancer.
In an older or middle aged cat the more common diseases we see include hyperthyroidism (a tumor of the thyroid gland that overproduces thyroid hormones) or if he were drinking more and urinating a tremendous amount, diabetes. Sometimes early on with diabetes we only see weight loss.
Both diabetes and hyperthyroidism are treatable or at least manageable.
Hyperthyroidism is controlled with daily oral medication (Tapazole, also called methimazole) or a very special diet called y/d both of which must be done for the rest of her life or treatment with radioactive iodine to selectively kill tumor cells which is usually a one time therapy. There are pros and cons to each therapy which your veterinarian can discuss with you after getting a firm diagnosis. The diet may be difficult for you as you have multiple cats, and if he eats any regular food it won't work. Likewise a normal cat that eats y/d can get very sick.
Hyperthyroidism is not painful, but elevated thyroid hormones can make them feel edgy or nervous and thus cranky, increase the heart rate and interfere with sleep, like drinking way too much coffee or energy drinks. Initially many owners may only see weight loss. Long term if left untreated it can lead to heart disease, hypertension, possible eye damage as well as kidney and liver damage. But if caught and treated many of these things can be reversed.
Diabetes is controlled with diet and insulin therapy.
Has he had any testing done to achieve a diagnosis?
Ideally he should have a complete blood count and biochemistry profile with T-4 checked as well as a urinalysis. These tests often give us the information that we need.
If those aren't diagnostic then it is time to move on to more specialized testing. I recommend checking vitamin B levels to look for signs of intestinal disease, a test for pancreatic insufficiency called a TLI, and an abdominal ultrasound to look for signs of changes in the walls of the intestines as well as evaluating his organs. He may need an endoscopy to collect biopsies of his intestines if they look abnormal.
In the meantime you might try feeding him a higher calorie prescription food called Hills a/d or Iams Maximum Calorie from your veterinarian and make sure he has access to plenty of fresh, clean water.
Please let me know if you have any further questions.