Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that you have been waiting for a response, but your requested expert isn't online which delayed your question coming up on the list for all to answer. I would like to help if you are still interested in an opinion.
I understand that your older kitty isn't eating and is drinking poorly compared to normal.
A loss of appetite can be related to eating something he should not have, too many treats or table food, eating foreign material (which could cause a full or partial gastrointestinal obstruction), chronic pancreatitis, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, inflammatory bowel disease, heartworm disease, internal organ failure, or even infiltrative cancers such as lymphoma.
Because he hasn't eaten normally in several days I am very concerned about him. If he isn't drinking well either he will quickly become dehydrated. As her liver breaks down fats for energy to live it can become overwhelmed and unable to function and perform its normal tasks, and he may develop a type of liver disease called hepatic lipidosis.
Ideally he would see a veterinarian if this goes on for more than 72 hours. Simple stomach upset should pass within 24 to 48 hours. They could examine him, run blood tests and possibly check radiographs and/or an ultrasound to evaluate him and know best how to treat him.
In the meantime they can administer injectable anti-nausea drugs and fluids to rehydrate him.
If you cannot have him seen for whatever reason there are some things you can try at home.
At home to try and settle his stomach and get him eating you can give either:
1) Pepcid ac (famotidine) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours
These are acid reducers and may help him feel less nauseous so that he will eat. They are quite safe and can be used long term if necessary.
You can use a medicine syringe to try and force water into him orally.
A couple of hours after giving the acid reducer I recommend offering warmed, meat baby foods or a bland diet mix of 2/3 boiled, minced, white skinless chicken and 1/3 boiled, white rice mixed with some warmed low salt chicken broth to make it easy to lap up and swallow, in case he has a sore throat or gums and teeth. This is also a great way to get additional fluids into him.
If this works and he starts eating well slowly mix in a canned senior cat food, and transition him to that.
If he continues to not eat he should see his veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.