...some docs get irritated when patients suggest what tests should or should not be done. In other words...they like to be the ones calling the shots (sometimes literally! ).
You have the right mindset with "he's not the only endo in town". I firmly believe that the doctor/patient relationship should be a partnership, which means that the two of you work together to attain the best possible health for you, not a dictatorship where the doctor lays down the law and you meekly follow along. I know a lot of doctors prefer the latter style because it saves them time; they don't wind up having to have long discussions with their patients and explain why or why not something is needed but it's really in the best interest of the patient if they understand as much as they can about their disease. Compliance is much greater when the patient has a good understanding of why they need to do what they need to do.
Give him the benefit of a doubt. Perhaps if you discuss this with him face to face and lay out your concerns, he'll be more agreeable to order the test. I don't think any doctor in his right mind would want to knowingly burn out his patient's pancreas or make his condition worse. It might also help if you bring along some literature showing why it would be a good thing to get checked.