Hello and thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
The symptoms that you describe are not specific for any one disorder and I would not make any long term insulin dose changes based on that. Refusal of a type of food ban be secondary to various causes including GI upset, pancreatitis, IBD, and other organ dysfunction. Sometimes, they just get tired of the fod. That being said, when you are dealing with an insulin dependent diabetic, whenever they are taking in less than normal calories, it is sometimes prudent to decrease their insulin temporarily until we can identify the reason or the decrease in appetite and correct it. I will sometimes have my clients skip a dose or give a reduced dose of insulin if not all of the meal was ingested. Remember in the short term, it is always safer to let their blood glucose levels run high than too low. Also if you are ever concerned that they are too low and do not have the ability to monitor BG levels at home, it is always safer to give a snack or a small meal of something they will eat or even some pancakes syrup or honey if they won't eat.
I would definitely contact your vet for a physical exam and advice on if his insulin dosing needs to be changed for the long term.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.