Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned for your fellow as he seems to be eating less, vomiting frequently and his stools have decreased in size.
If he is vomiting most of what he eats, and eating less, there will be very little left to make stools, so the smaller stools are not surprising.
Vomiting and a decrease in appetite can be related to something as simple as a quick change in diet or treats or getting into something he should not have like the garbage, a bug, plant material or a toxin. I understand he is not a fellow that has access to getting into things he shouldn't.
More serious causes of vomiting and loss of appetite are viral or bacterial infections, gastrointestinal parasites including worms and protozoal parasites, chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, inflammatory bowel disease, internal organ failure, a full or partial gastrointestinal obstruction, and abdominal mass causing pressure on his gastrointestinal tract, or even infiltrative cancers like lymphoma.
While I cannot guarantee that he will be improved by seeing a veterinarian, as we don't know why he is sick and he is an older fellow, his chances of improvement are much better if he sees a veterinarian. They can rehydrate him, give him injectable medications to soothe his nausea and irritated gastrointestinal system and perform some diagnostics to find out why he is sick thereby coming up with the best treatment plan for him.
In the meantime at home to try and settle his stomach today 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 half of 20mg tablet per 20 to 40 pounds of body weight every 24 hours.
These will reduce stomach acid and should help if this is related to simple nausea and gastrointestinal irritation. These medications can be used for several days as needed as they are quite safe.
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. Small amounts of water or ice cubes given frequently are fine as he needs fluids after all that he has lost with vomiting. You can give him pedialyte to replace electrolytes too but Gatorade is much too high in sugar which can make his intestinal irritation worse. Do not let him drink too much at any one time as that can lead to further vomiting.
After his food fast start a bland diet of 1/3 boiled, lean hamburger (or boiled, white skinless chicken), all fats drained off the meat, mixed with 2/3 boiled, plain white rice. Feed small meals frequently until you see out that it is sitting well with him, then gradually increase the amount and decrease meal frequency. I would start with 1/4 to 1/2 cup for the first meal.
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.
If he develops a tense painful abdomen, continues to vomit even with the acid reducers, becomes very lethargic, or runs a fever greater than 103.5F or has a subnormal temperature (less than 99.5F) then he really must be seen by a veterinarian as soon as possible.
If he continues vomiting even water now and feeling poorly though it would be best that he see a veterinarian on an emergency basis now as anything you give him orally will just come back up worsening his dehydration.
If his vomiting continues I would recommend checking basic blood tests on him including a complete blood count, biochemistry panel and a blood test for pancreatitis called can spec PL (canine specific pancreatic lipase) which is highly specific for pancreatitis. If his electrolyte levels seem unbalanced (high potassium and low sodium) then testing for Addison's disease with a test called an ACTH response test.
If those things are normal then the next step diagnostically would be an abdominal ultrasound when he is showing symptoms and endoscopy to collect intestinal biopsies.
Please let me know if you have any further questions.