Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your little one Rufus isn't feeling himself today, vomiting yesterday, lethargic, refusing to eat today, drinking little, and just not himself.
Vomiting and refusing to eat is not indicative of any disease process but tells us he is quite nauseous.
Is he running a fever (rectal temperature greater then 103F)?
Does he have a tense, painful abdomen with gentle pressure on his belly with your hands?
Vomiting and nausea can be due to stress, a change in diet, parasites, a viral or bacterial infection, eating something he should not (like a piece of a toy or string), inflammatory bowel disease or even a food allergy. But it can also be related to metabolic organ disease (such kidney or liver disease) due to organ wastes irritating the gut or an obstruction in the gastrointestinal tract.
Please check the litter box to make sure that there are the normal amounts of urine being passed. Male cats can get urethral obstructions due to inflammation, mucous and crystal formation. If they cannot urinate they quickly go into kidney failure due to back-pressure on the kidneys from being unable to urinate and that can cause vomiting and abdominal pain. If he is quite uncomfortable with abdominal pressure, has been back and forth to the litter box several times today, and you cannot tell whether he is urinating normally today an emergency veterinary visit is best.
But if he can urinate fine then there are some things we can do to try and settle his stomach.
To try and settle his stomach today you can give either:
1) Pepcid-ac (famotidine) at a dose of one quarter of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one quarter of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours.
These will reduce stomach acid and should help if this is related to simple nausea and gastrointestinal irritation. Either one can be given for a few days if need be.
I would wait to feed him any food for a good 6-12 hours after the acid reducers are started. This should help stop gut spasms and restore normal gut motility. Small amounts of water or clear broths like low salt chicken broth offered frequently are fine as he needs fluids. You can give him pedialyte to replace electrolytes too, though most cats don't like the taste.
After his food fast offer him a bland diet mix of 2/3 boiled, white skinless chicken, all fats and juices drained off the meat, mixed with 1/3 boiled, plain white rice. Feed several small meals a day.
Once he feels better (no retching for 48 hours and eating the bland diet well) start mixing in his regular cat food very slowly, less bland more regular with each day. It should take a week or so to convert him back.
If Rufus is continuing to vomit and not wanting to eat for more than 48-72 hours, runs a fever or has a tense painful abdomen and is feeling poorly though it would be best that he see a veterinarian now as anything you give him orally will just come back up worsening his dehydration. Because he isn't eating or drinking normally I am very concerned about him. He will quickly become dehydrated and as his liver breaks down fats for energy to live he may develop a type of liver disease called hepatic lipidosis.
Best of luck with Rufus, please let me know if you have any further questions.