An internist or endocrinologist would be appropriate doctors for diabetes care, but it sounds like your father needs to see a registered dietitian who would be the most appropriate to provide diabetes nutrition therapy. Dietitians have the education and experience to provide nutrition therapy using a person’s medical history. It would probably be very helpful to have an eating plan to follow with recommended foods. Your father needs a diabetic, low-fat and carbohydrate- controlled, no alcohol meal plan.
By the way, there is no such thing as borderline diabetes. It’s like saying a woman is borderline pregnant. Either your father can eat all the carbohydrates he wants and his blood glucose will return to normal levels within 4 hours or he has to limit carbohydrates and possibly take medication. Just because a person with elevated blood glucose doesn’t have to take medication to control diabetes, they still have diabetes.