In many cases a poor appetite and vomiting are triggered by eating something they should not, too much table food, too many treats or something they find outdoors.
More serious causes in older dogs include viral or bacterial infections, chronic pancreatitis, a dietary allergy or sensitivity, inflammatory bowel disease, internal organ failure (kidney or liver disease), ketoacidotic diabetes, a full or partial gastrointestinal obstruction, an abdominal mass placing pressure on his gastrointestinal tract, or infiltrative cancers such as lymphoma.
In his case with normal bloodwork an infection, or organ failure are both very unlikely.
If he has not been tested specifically for pancreatitis I would do so.
At his age I would also be very concerned that he has a mass in his abdomen that is at least partially blocking passage of ingesta through his gastrointestinal tract making him feel full and nauseous. Sometimes we can see the mass on radiographs but because tumors are often soft tissue they blend in to the tissues around them and unless they are very large we may not see them on radiographs. Smaller, infiltrative type tumors may not be seen on radiographs, we may need an ultrasound to see those.
I recommend trying an over the counter acid reducing medication that people take for acid reflux to see if we can ease his nausea. You can give either:
1) Pepcid ac (famotidine) at a dose of one 10mg tablet per 15 to 40 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of 1/2 of a 20mg tablet per 15 to 40 pounds of body weight every 24 hours
3) Ranitidine (Zantac) at a dose of 1/2 of a 75mg tablet per 35 to 70 pounds of body weight every 8 hours
These are acid reducers and any of them could help him feel less nauseous. They are quite safe and can be used for several days if necessary.
I would pick up all food for now and water for a couple hours to allow his stomach to settle after the acid reducer dose.
In a couple hours when you give him water or clear broth make sure it is in small amounts only. If he drinks too much too quickly that can lead to vomiting. To get some electrolytes in you can also offer him a 50:50 mix of pedialyte and water.
If there is no vomiting for 8 hours offer a bland diet of 1/3 boiled minced white skinless chicken or white fish, or lean hamburger and 2/3 boiled white rice mixed with some low salt chicken or beef broth to make it soupy and more palatable. If he refuses that you can offer a little meat baby food.
Hopefully he will be able to hold that down until he can be rechecked.
If he continues to vomit even with the acid reducer and bland diet, runs a fever (more than 103F rectally), has a tense painful belly or if he refuses to eat he should see a veterinarian on an emergency basis for a recheck examination, diagnostics, injectable anti-drugs intravenous fluids and supportive care.
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