Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Taki is refusing to eat, drinking a lot of water and vomiting yellow, bile material.
His increased water consumption may indicate underlying metabolic organ failure, especially if this started before the loss of appetite and vomiting and he has been producing more urine then usual. For example kidney failure and elevated kidney enzymes lead to a drive to drink more, nausea and vomiting.
Does your fellow have a good stream when he goes outside to urinate? Older male dogs can get prostatic enlargement and infections which can place pressure on their urethra making it difficult to empty his bladder, leading to back pressure on his kidneys and secondary damage.
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, esophageal reflux, 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.
Is he passing any stools? If not this could be related to an obstruction.
At this point with several things going on a hands on veterinary examination, urinalysis and blood work including a complete blood count and biochemistry profile is indicated as soon as possible. I understand if he cannot be examined today given that it is Easter, so in the meantime it may also be helpful to put him on an acid reducing medication to try and decrease his nausea/vomiting.
I recommend 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 with-hold food for about 4-6 hours after one of the acid reducers is given to make sure he is no longer vomiting.
If he stops vomiting you might also consider feeding a bland diet consisting 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 and get additional fluids into him. If he refuses that you can offer a little meat baby food.
If he refuses both then don't push it, he needs hands on veterinary care.
But if things go well and he does eat and doesn't vomit feed him the bland diet for 2 to 3 days then slowly start to mix back in his regular food, a little more at each meal. It should take about 5 to 7 days to slowly convert him back to his regular diet.
If he continues to vomit even with the acid reducers, runs a fever (more than 103F rectally), has a tense painful belly or if he refuses to eat she 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, as well as a urinalysis.
Please let me know if you have other details or a particular question based upon my response.