Thank you for the additional information. It is very dangerous to fast a kitty as they can experience severe organ dysfunction if they go too long without eating, so don't withhold food at all. The only time it is adviseable to keep food out of reach is if your kitty starts to vomit and cannot stop, and you should seek veterinary assistance immediately so that the vomiting can be controlled.
Decrease in appetite and water consumption and lethargy are all major symptoms of illness and your kitty will need to be examined by a vet ASAP, preferrably tomorrow. Cats are considered to be geriatric at 8 years old and can be prone to illness as they age. The 3 most common ailmenst known to afflict elderly cats are diabetes, kidney disease and hyperthyroidism. The symptoms include vomiting, diarrhea, increased water consumption, increased urination, dehydration, weakness, weight loss, muscle wasting, poor hair coat, changes in appetite, changes in behavior and increased vocalizations. I suspect that your kitty is dehydrated, and may be malnourished as well. Cats instinctively hide illness, and once they are exhibiting obvious symptoms they are usually suffering from advanced stages of illness. Your kitty needs to see a vet for an exam, comprehensive blood and thyroid panel and a urinalysis to evaluate organ function and hydration status and screen for illness. Once you are seeing signs of illness it is smart to have it addressed sooner rather than later as it can save your kitty alot of discomfort and you alot of money.
Please have your kitty seen tomorrow so that he can start to feel better. Please let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. Good luck!