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Hi this is Dr. Hill. Please give me a moment to type my initial response. Thank you.
I need to ask a couple of questions to better help you with Lion.
Is he indoor only?
Do you have any other cats in the household?
Would you be able to check his rectal temperature at home?
Lets start there.
Okay, thank you. There are a number of possible causes of his lethargy, loss of appetite and difficulty getting around. In a male cat that is not eating as much or urinating normally, I am most concerned about an issue with the urinary tract. Males can become quite sick very quickly if they are not able to urinate. It can either be caused by a urinary tract infection or inflammation (cystitis). If he is going in and out of the litter box with minimal urine production, then he needs to be taken to an emergency clinic today because an obstruction can become life threatening.
The less critical concerns would be with osteoarthritis in his back legs/hips or pain/nausea that is causing a loss of appetite and discomfort with jumping. I recommend that you check his rectal temperature. Normal temp for a kitty is 99 to 102.9. If it is over 103.5 then he should be taken to the emergency clinic today. If the temperature is normal and you can coax him to eat with some tuna fish or canned cat food, then waiting until tomorrow to see your local veterinarian is not unreasonable. Your local veterinarian will likely recommend some baseline blood work and a urinalysis to fully understand what is making Lion feel so lethargic. X-rays of his pelvis would not be unreasonable, but this would be based on physical exam findings. Please ask any follow up questions. Cats are very good at masking disease, so definitely get him looked at tomorrow if possible. If you are satisfied with my response, then please take time to rate my response. You can continue posting on this thread for several days, even after rating. Happy New Year.