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I am sorry to hear about your cat. I have never been lucky enough to have a cat live so long. I am a veterinarian and will do my best to help.
If these symptoms are due to a bacterial infection, then antibiotics should help. Many upper respiratory infections are viral infections, and they do not respond to antibiotics. It may be worth a try; if antibiotics are going to help, you should see improvement in a couple of days.
Putting him in the bathroom when it is hot and steamy may help with his congestion. You can also use childrens nose drops, like Little Noses, once to twice daily for nasal congestion.
Are you giving him supplemental fluids under the skin for his kidney disease? That may help his kidneys and his congestion.
Let me know what else I can help with.
Thank you for letting me know. My sincere condolences. I hope his passing was peaceful.