He said it was unusual for a 15 yr old cat that was raised on raw meat to be diagnosed with diabetes. He thinks it's a pancreatic tumor!
I despise vets that give that kind of "diagnosis" without any proof at all.
The biggest risk factor for developing diabetes is heredity...if diabetes runs in the family, you're much more likely to become diabetic....but I doubt if you (or this vet) knows anything about Ravan's parents, grandparents, great grandparents, etc.
Diet plays a role too, but not as much as heredity does. You'll hear all the time "my cat ate nothing but cheap kibble all it's life and lived to be 150" (well, not really but you get the idea

)
Overweight cats are more likely to become diabetic but it's only part of the picture.
No matter what you feed, if diabetes runs in the family, you're more likely to have a diabetic cat....period.
Do cats get pancreatic cancer? Yes, they do....but it's not terribly common so at this point, it's not something I'd lose sleep over.
We do need to get him eating though....have you ever tried one of the appetite stimulants like Cyproheptadine or Mirtazapine?
Does he show any signs of nausea, like sniffing at the food like he's hungry but then walking away? Lip-licking?