Some dogs develop a condition called exocrine pancreatic insufficiency, (EPI) after pancreatitis. The most distinct symptoms are greasy, grey cow pat poos or diarrhoea, weight loss and ravenous appetite. Vets are not familiar with it, as I know too well from personal experience. The dog will not be producing enzymes to digest food, so the basic treatment is to supplement with replacement enzymes.
If this is relevant to you, you would need to ask the vet for an epi blood test, B12 and folates. It’s not cheap , but is definitive. Once diagnosed , and given the proper lifelong treatment, they should return to normal .
Information is on epi4dogs website. It isn’t a quack setup.
Gut problems are soothed with slippery elm or Dorwest bark. Is there any blood is the poo? Has he got acid reflux? I can’t comment on the Hills food. I feel very strongly that you should see your vet asp. and check for epi. Don’t agree to any invasive tests like endoscopy or biopsies until you’ve checked this.
If you aren’t satisfied with your vet, try a different one.
Feel free to send me a private message if you need to know more. I’m not a vet, but lived with a spaniel with epi for eight years.