I am no expert in the field but, from my experience, food and water is usually completely stopped from 1-5 days to let the pancreas
REST. Fluid therapy, regular blood work (sometimes several times a day), pain
meds or a patch for pain and antibiotics are often used. This is a very painful condition. In pancreatitis, nutrients the dog needs are passed out in the feces, thus probably the reason for the weight loss (and that is quite a bit). Has he been checked for diabetes?