Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that your fellow Chester isn't feeling well and is eating poorly for the past week.
A decrease in 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, esophageal reflux, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, inflammatory bowel disease, heartworm disease, hyperthyroidism (a tumor of the thyroid gland) with secondary organ failure, internal organ failure, or even infiltrative cancers such as lymphoma.
Because he hasn't eaten normally in several days and he is likely not drinking normally either I am very concerned about him too. He will quickly become dehydrated, and as his liver breaks down fats for energy to live it can become overwhelmed and unable to perform normal functions, he may develop a type of liver disease called hepatic lipidosis.
Ideally he would see a veterinarian today, or tomorrow if you don't have access to veterinary care today, since this has been going 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 improve his appetite you can give either:
1) Pepcid ac (famotidine) at adose 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 better. 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 if he isn't drinking well, or you can offer chicken or beef broth or water mixed with tuna to get him to drink.
A couple of hours after giving the acid reducer I recommend offering meat baby foods or a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow.
If he continues to not eat normally he should see her veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.