I am sorry to hear about your fellow's repeated bile vomiting, lack of appetite and decrease in drinking despite therapy with fluids, steroids and antibiotics and I understand your concern.
If his blood tests and radiographs are normal and not diagnostic the next step would be an abdominal ultrasound and/or endoscopy with biopsies. But I understand given his personality that may be difficult.
It sounds like your veterinarian is treating him presumptively as a primary intestinal disease case.
Were his cobamalin and folate levels checked?
If he isn't responding to the medications he is taking we can add an antinausea drug including Cerenia (prescription from his veterinarian), an acid reducer medication such as Prilosec (omeprazole) or Pepcid (famotidine) which are over the counter drugs you can purchase at the drug store, and a prescription appetite stimulant called mirtazapine.
Today if you would like you can try either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours.
These will reduce stomach acid and should help decrease symptoms related to reflux, so they may help him. These medications are quite safe and can be used long term if need be.
Ideally he should be eating a prescription hypoallergenic food, but these aren't highly palatable and we want him to eat so instead you might try feeding him a higher calorie prescription food called Hills a/d or Iams Maximum Calorie from your veterinarian and make sure he has access to plenty of fresh, clean water.
Rarely a parasite called a stomach worm (Physaloptera) can lead to persistent, intermittent vomiting. These are difficult to diagnose because they rarely shed eggs, so we can try simply worming him with a gentle wormer called Pyrantel and see if that helps. This is easy to do and inexpensive so well worth trying.
Please let me know if you have any further questions.