I am sorry to hear about all of Suede's problems. I am a veterinarian, and will do my best to help.
Is his blood sugar controlled, less than 200 or so, on the 6 units of insulin twice daily? He will not gain weight, no matter what he eats, if his blood sugar is not under control.
If he cannot eat fat or carbohydrates, that only leaves protein and fiber. Fiber can help control his blood sugar, but will not encourage weight gain. Is he on a high protein, low carbohydrate, low fat diet such as Evo, or Royal Canin Diabetic Diet or a lowfat grain free diet? What is he eating now?
The other alternative would be to cook for him: peas, lean meat and fish, low fat dairy products (if he is not lactose intolerant).
He may be metabolically unable to put weight on, with his problems. If his blood sugar is uncontrolled, over 400, he can't gain weight no matter what he eats.
If his pancreas is not making the enzymes needed to use the food he eats, you might consider adding pancreatic enzymes like Pancreazyme, or feeding him fresh frozen pancreas from a butcher.
Let me know what else I can help with or answer. Rebecca