If I remember rightly, this issue has been raised in relation to the dog pancreatitis - can't remember if it's in the FB group or Tully's specialist.
I assume you had a full panel to check liver, kidney, diabetes, pancreas? There is another more precise blood test, that you'd need to send away for, that would show if he has underlying chronic pancreatitis. Given the meds he has been on and his age it could be. I'm just trying to dig up Tully's bloods. She had to go on a very low fat diet to deal with it.
UPDATE - Tully's chol was high and triglycerides even higher. Diagnosis was hyperlipidemia possibly due to chronic pancreatitis. It has dropped since she's been on pred and RC Gastro Low Fat. Just OT - her most recent tests showed very low serum glucose. I'm surprised the IMS didn't tell me, no wonder she had no energy!