The major choices or differential diagnosis for what you describe are: Cushing's Syndrome, Addison's Disease, Hypothyroid, Cognitive Dysfunction, Neoplasia.
Even if your dog has tested negative for any of the above listed conditions, he may still have one or more conditions, but they could be atypical or may not as yet be detectable in the blood. Mottling of the pancreas could be due to inflammation, scarring from prior bouts of pancreatitis, or neoplasia.
If a definitive diagnosis cannot be found than possible therapeutic trials with safe and innocuous medications may be worth a try. Please discuss this with your veterinarian.