Hi there and thanks for your question.
I just have a few questions for you before we get started:
There are a few things that can cause the symptoms you mentioned.
My first concern would be a urinary tract blockage which can cause very severe disease quite quickly.
While a cat bite wound causing a fever could do this it would be a little unusual for him to be this drastically sick so suddenly.
If he can't walk there is a possibility of a spinal issue such as a back injury or a tumor affecting the spine or a prolapsed disc.
It is also possible that he has a blood clot affecting the flow of blood to his back legs. If this is the case you will notice that his back feet feel a lot colder than his front feet.
There are a multitude of other possible problems in an older cat.
Regardless, this cat really does need to be seen by a vet today. There really isn't much I can suggest for you to do at home other than to bring some food to him to see if he will eat, but this is not likely given the symptoms he is displaying.
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would highly advise contacting your regular veterinarian.
It does sound like this is either a blood clot or a serious back problem. Unfortunately often there is no cure for these. Sometimes with a back problem if we treat it early enough we can reduce inflammation and resume feeling.
Your boy really does need to be seen by a vet this morning!