If overweight, then gaining weight is not a good thing, as it will adversely affect the pancreatitis
It's curious that he's got pancreatitis
if he's been on a low-fat diet for 3yrs. The blood result that points directly to it is the amylase, which has been consistently high on both blood results. I would raise this with your vet and please let me know what he suggests (cause, diagnosis and treatment/management advice).