Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am so sorry to hear about Spooky's lack of eating, vomiting and generally not feeling well.
Has he lost a significant amount of weight?
What are his stools like?
Chronic vomiting can be related to chronic pancreatitis, a dietary allergy or sensitivity, inflammatory bowel disease, heartworm
disease, internal organ failure, hyperthyroidism
, a full or partial gastrointestinal obstruction or even infiltrative cancers such as lymphoma.
I am glad to see that he's had some blood testing done. I would check with your veterinarian to make sure that those tests included a heartworm test, as vomiting is strangely one of the most common signs of heartworm disease in cats. I would also check to make sure a specific blood test for pancreatitis has been done, a fel spec PL (feline specific pancreatic lipase). This test is highly specific for pancreatitis in cats.
I would also check that his thyroid hormone levels have been checked.
If those tests have been done or are done and come back normal the next step is an abdominal ultrasound and/or either exploratory surgery or endoscopy to gather biopsies of his gastrointestinal tract to look for inflammatory bowel disease or cancer.
If he improved after the steroid injection I would be highly suspicious of heartworm disease, inflammatory bowel disease or lymphoma as all respond to steroid use.
In the meantime you can try giving him an acid reducer to see if that helps. You can try either:
1) Pepcid ac (famotodine) at 1/4 of a 10mg tablet per 5 to 10 pound cat every 12 to 24 hours
2) Prilosec (omeprazole) at 1/4 of a 10mg tablet per 5 to 10 pound cat every 24 hours.
These acid reducers can help with nausea, are quite safe and can be used long term if necessary.
You could also try feeding a canned only sensitive diet such as Hills i/d or Purina Veterinary Diets EN. If you've tried these sensitive stomach diets then perhaps a hypoallergenic diet such as Hills z/d or Royal Canin Duck and Green Pea.
If you are unable or unwilling to have further diagnostic testing done and are considering euthanasia due to his condition then treating with steroids and a low allergen/low irritant diet long term is reasonable to keep him as comfortable as possible as long as possible, trying to work him down to the lowest dose possible that controls his disease.
Please let me know if you have any further questions.