Ditto also on the above posts with a vet visit. Once quick way you can get a possible hint as to what is going on is to smell his breath. If there is an ammonia odor then his kidneys are not flushing as well as they should. Check for dehydration because even if you THINK he is getting enough water as they get older they can always use more. Especially as the kidneys start to slow down.

Good luck. Let us know what happens.

Denyce