May I suggest that you get a stomach fluid sample to check for pH and bacteria? That was the big problem for us, and it created the intestinal inflammation. I had to treat with enrofloxacin for about 2 months, along with steroids, to get to improvement. As long as he is at vet hospital, a quick gastroscopy to pull fluids will help give answers to the ‘why’. Also, what’s the steroid dose?
ETA - if your vets disagree with the bacterial testing, and you’re at the end of the rope, please please do try an antibiotic like enrofloxacin. If there is no other option on the table and you’re grasping for straws, this may make a difference. I would literally beg them at that point since you’d be otherwise out of options.