If you truly love cats, take the cat to the vet and spend the $200 to get him back in shape again.
Yes, that is exactly what I would do/ have done/ more than once or twice.
Bryan and I spent $800 on a tiny old cat that someone abandoned, but we couldn’t save her.
We have her ashes, along with the ashes of all our late cats.
Our cats do not go outside, ever.
My late Rocket was the exception, but he only went out with me by his side the entire time.
No, he didn’t need a leash. He just stayed by me. We were close. I miss him and many others.
((Hug))