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Take your cat to the vet ASAP. He is likely very ill. Not drinking for several days is very serious because he is likely now dehydrated. Accompanied with the lethargy and lack of appetite points any number of major illnesses. Illnesses in elderly cats can progress rapidly and should be caught as early as possible. He needs to be seen by a doctor and diagnosed ASAP as he seems to be fading fast!
Hope this helps!
It could have been lymphosarcoma if he had a spinal tumor it can cause the paralysis, lethargy, loss of apetite:
Because he was an outside cat at times he may have eaten something poisoness outside as well.
It could have been heart disease:
Or it could have just been old age or it could have been a combination of things. Without him having been able to get to a vet and without any idea of what may have triggered his illness it is hard to give an accurrate diagnosis. One thing though that you can know for sure is that for elderly cats it is important to take them to the vet as soon as they show signs of illness because of their weakened state illness can progress very quickly.