Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your older fellow Reilly vomited last night and despite not eating or drinking has vomited yellow material today.
Yellow in the vomit means that the small intestine is refluxing bile into the stomach so that when he vomits you see the yellow color. That isn't normal as bile doesn't belong in the stomach, and it does mean that there is some reverse motility, but it isn't specific for any particular disease process.
In many cases vomiting is triggered by eating something they should not, too much table food, too many treats or something they find outdoors, or a dietary allergy or sensitivity. More serious causes of vomiting include viral or bacterial infections, chronic pancreatitis, inflammatory bowel disease, internal organ failure (kidney or liver disease), a full or partial gastrointestinal obstruction or even infiltrative cancers such as lymphoma.
Because of his age and refusal to drink, along with continued vomiting I am very concerned about him. He may be becoming dehydrated. Ideally he would see a veterinarian today if possible.
I realize that may not be possible given it is later in the day.
For now to ease nausea it may also be helpful to put him on an acid reducing medication as too much stomach acid is very irritating and predisposes to vomiting.
I recommend giving him a dose based upon his weight. You can give either:
1) Pepcid ac (famotidine) at a dose of 1/4 of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of 1/4 of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours
These are both acid reducers and may help him feel better. They are quite safe and can be used for several days if necessary.
I would not feed him any food for 12-24 hours after the acid reducers are started. This should help stop gut spasms and restore normal gut motility.
Starting an hour or two after the acid reducer is given small amounts of water or ice cubes offered frequently are fine as he needs fluids after all that he has lost with vomiting. You can give him pedialyte to replace electrolytes too to replace electrolytes.
If the vomiting stops in the morning after his food fast start a bland diet of 1/3 boiled, lean hamburger (or boiled, white skinless chicken), all fats drained off the cooked meat, mixed with 2/3 boiled, plain white rice. Feed several small meals a day.
Once he feels better (no vomiting for 48 hours) start mixing in his regular dog food very slowly. Less bland more regular with each day. It should take a week or so to convert him back.
But if he continues to vomit even with the acid reducers and bland diet, runs a fever (temperature more than 103F rectally), has a tense painful belly, or if he refuses to eat tomorrow he should see a veterinarian for an examination, and some diagnostic testing.
I would start with a complete blood count and biochemistry profile, a specific test for pancreatitis called canine specific pancreatic lipase, and possibly radiographs of his abdomen. While they wait on test results he can be given injectable medication to sothe his nausea and intravenous fluids to rehydrate him and rebalance his electrolytes.
Please let me know if you have other details or a particular question based upon my response.