Hello, my name is***** and I have over 20 years of experience as a veterinarian.
It sounds like your fellow has had a poor appetite and minimal water consumption and has been lethargic for the past few days.
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, Addison's disease (hypoadrenocorticism), inflammatory bowel disease, or even infiltrative cancers such as lymphoma (unlikely at his age).
In Junior's case because he passed a plastic bag I am concerned that there may be more foreign material stuck in his gastrointestinal tract, he has had significant damage to his gastrointestinal tract when pushing foreign material through and now has severely damaged intestines, or a secondary bacterial infection/peritonitis.
Because he hasn't eaten normally in several days and he isn't drinking, and is so weak and lethargic I am very concerned about him. He will quickly become dehydrated.
Ideally he would see a veterinarian today given his condition and the length of time this has lingered. Simple stomach upset should pass within 24 to 48 hours, and he should have perked up and felt much better after the bag passed if that were the only problem. 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 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 quarter of a 20mg 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 foods 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.
If he continues to not eat, runs a fever (rectal temperature greater than 103F) or has a subnormal temperature (rectal temperature less than 99F) he should see a veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.