Older Cat Not Eating But Drinking. This is very safe and virtually has no side effects. Help with gentle grooming if your cat will let you.

Hopefully this is just caused by the stress of your cats changed environment due to the loss of a companion and a new one introduced into the household. That was about a week and a half ago. Cats develop a condition called feline hepatic lipidosis after a fairly short time with no food in which they get fatty deposits on the liver.
One way to do this is to offer your cat multiple drinking stations.
He will only drink water and a bit of milk his pupils have. Encourage food consumption and hydration if your cat doesnt eat for 24 hours or doesnt drink for 12 hours. However it will be the frequency and severity of these conditions that determine your cats overall state. Losing a beloved family member whether animal or human can cause some cats to stop eating due to anxiety or depression.

