Exactly as petallus has described for our last cat ( virtually a twin of your cat Linda) and in the end for all the cats we had,they run up to the food bowl, crouch over it and only have a mouthful or none at all.They drink a lot of water. It's sad, but if the quality of life goes,there isn't much to be done.
vets try and keep them going, hydrate them and advise tablets, but it's just prolonging the inevitable, in any case trying to give cats a tablet every day is beyond difficult.I would take him to the vet,find out if there is an obvious cause or injury, and take it from there.