Thank you Jason,
First, I share your concern about what sounds to be quite dramatic weight loss for Pepe without an obvious source (ie dramatic appetite decline, or loss via severe vomiting or diarrhea). And if he is suffering from an internal issue that is causing a dramatic siphoning of nutrition from his normal body systems, then it is not uncommon to see loss of coat quality, loss of muscle mass, and even weakness.
The hair/coat situation is a sign that despite his great appetite, his body is struggling to meet its general maintenance nutrition requirements (therefore it is diverting nutrition for less critical systems like the hair/coat and likely redirecting these to more important organs). Furthermore, if his body is struggling for nutrition such that we are seeing this and weight loss, then it is not unexpected to see him feel weak or be also experiencing loss of muscle mass in places like his hips and back legs. With the muscle atrophy, this is essentially due to the body using its own resources to again support basic and critical life functions. So, these are all red flags that something is siphoning his body resources from him despite his great appetite.
With this in mind, we have to appreciate that this isn't an uncommon scenario in cats around his age. We can see older cats start struggle to keep weight on despite a good to great appetite for a range of reasons. Issues that can manifest this way at his age will include conditions like hyperthyroidism, diabetes, liver disease, and kidney troubles. As well, while it isn't nice to think about, we must keep in mind that cancer in cats his age can manifest as weight loss with little other signs (just as it doesn't impinge on the body, only to steal nutrients from him). As you noted, worms can also cause there signs but often dramatic weight loss due to worms is a feature of kittens and not commonly seen in adult cats.
To get to the bottom of what is going on it would be worth having Pepe checked by his vet this week (if he is due for a vaccination soon, you could move it up a wee bit early and have him checked out at that time). The vet will be able to have a feel of him and just make sure there are no sinister lumps and bumps to blame for his weight loss. And if you were able to bring in a urine sample at that point, the vet could check it for signs of diabetes (ie. sugar in the urine) as well as check its specific gravity (how concentrated it is) that can tell us if there are problems with his kidneys. Urine samples can often be collected by placing the cat in a carpet-less room with an empty (or with non-absorbable litter) litter box overnight.
If hyperthyroidism is a potential after your vet has examined him, then a blood sample may be checked to assess his thyroid hormone to determine how severely affected he may be. As well, if you do have a blood sample checked, you can also have his blood sugar, liver and kidney parameters checked at the same time, giving you a good chance of ruling out the above differentials and determining the cause of your wee one’s abnormal weight loss.
Overall, in his case it would be prudent to have him checked out and at least have a urine sample (or blood sample) checked. Because all of his signs tell us that one of these issues are lurking and stealing nutrition from him. And if you are able to the bottom of the exact cause sooner rather then later, it will give you the best chance to deal with it and help support him in a way to make him more comfortable and possible regain some weight.
I hope this information is helpful.
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All the best,
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