Have Cat Questions? Ask a Cat Vet Online.
Hi this is Dr Hill. Please give me a moment to fully type my response.
If Whiskers is very thin, then he likely has a medical issue such as diabetes, hyperthyroidism, kidney disease or cancer. He needs to be rechecked by your local veterinarian so that they can run some bloodwork to check for those issues. Milk can cause diarrhea in some animals, so best to stop. Try to offer him food more frequently throughout the day, until you can see your local veterinarian. Feel free to post follow up questions.
that sounds like diabetes or hyperthyroidism. They are typically ravenous and losing weight.