Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that Levee has had a poor appetite, isn't drinking, and has been lethargic for the past today. While I don't think he needs an emergency visit tonight I think seeing his regular veterinarian tomorrow is best.
Lack of an appetite and lethargy 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, organ failure (kidney or liver disease), Addison's disease (hypoadrenocorticism), inflammatory bowel disease, or even infiltrative cancers such as lymphoma or an abdominal mass taking up space in the abdomen so he feels full are all possible causes.
Ideally he would see his veterinarian tomorrow unless he is feeling much better. 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.
Tonight 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 one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 24 hours
These are acid reducers and may help him feel less nauseous so that he will eat and drink. They are quite safe and can be used for several days if necessary.
A couple of hours after giving the acid reducer I recommend offering meat baby food or a bland diet of 1/3 boiled, minced, white skinless chicken or boiled, lean hamburger and 2/3 boiled white rice mixed with some low salt chicken or beef broth to make it easy to lap up and swallow.
But if he continues to not eat he should see his veterinarian tomorrow for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.