As a diabetic, I can say that water is desirable at night because I get dehydrated. That being said, I am controlling my diabetes with diet, not insulin now. Have you looked at the food? A meat based food is going to do more to control diabetes than a grain based. I find that if I eat grains in the evening, I am much more thirsty at night and my sugar level fluctuates more. I would look for a food with chicken, beef, or other meat and very little rice or corn. Supplement with green beans, broccoli, or other vegetables if he will eat them. Just some suggestions