Hi Peter,
Sorry about your father. Patients with pc do have a hard time gaining and maintaining weight, You can make your own protein based shake for him. Start with fat free milk, use fat free cottage cheese for substance, artificial sweetener, fresh fruit such as strawberries or blueberries and protein powder you can buy at any health food store. Also offer him small amounts of food often during the day. Foods that are high in protein are scrambled eggs with cheese, avocado, peanut butter on crackers, almonds, fish, chicken and cheese. Keeping weight on is such a problem for pc patients that most docs advise that they eat whatever sounds good to them and they can keep down. The diabetes is considered secondary and elevated blood glucose can be dealt with if need be.
Pancreatic enzymes are also a good idea. However, they must be RX'd from his doc, not otc. They must be taken with any food because the pancreas is compromised and can no longer process the nutrients he eats. Take one or more as he begins to eat, then take one or more midway through the meal and last take one or more about 3/4 of the way through the meal. Alternatively, the enzyme capsules may be opened and the contents sprinkled on the food. Finding the optimum amount, different for each patient, will be a matter of trial and error.
Good luck.