I have a 9 year old male. He has always been moderately skinny and somewhat weak constitution. He'd get sick at least once a year for a week or so and need antibiotics, but fine otherwise.

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5 days ago he started getting sick again. Just some sniffles and mild lethargy. This has happened in the past, and while I noticed it, I honestly wasn't concerned. He never really improved though. He was sleeping like 23 hours a day. Saturday night he started hiding. Whenever he starts hiding it is a red flag to me that something is serious. He still slept next to me as usual, but Sunday he was even worse. He stopped eating or drinking period. Vet wasn't open on Sunday, so I figured I'd get him to vet first thing Monday morning. I got there early and had to wait 6 hours before I could be seen. They gave him subcutaneous fluids and drew his blood for a blood test. I got the results today and they told me hsi kidneys have failed and that I need to consider euthanasia soon. His levels are over 15 (I didn't know what metric, was stunned at the time. )

I'm at a total loss. I am very attached to my cat. I work from home, he typically sits in my lap half the day then sleep next to me at night. I don't understand how he went from totally fine 5 days ago to death's door today. I'm still in shock and confused. I know life isn't fair, but I'd figure there would be some sort of meaningful warning signs along the way. I don't know if the Vet is right. It is obvious my cat is very sick. He smells awful, can barely walk, won't eat at all, barely takes water; maybe one lick then done.

I don't know what to do. The vet implied there is no point in any treatment. I'm willing to do something if it has real chance of success, but I'm not interested in prolonging his life pointlessly. I'm confused because this is so sudden and doesn't appear to match stories I've read. His symptoms match a cat that's dying from CKD though. I'm leaning towards having him put down tomorrow, but wanted to solicit some advice from others first.

Opinions?