10 year old with diabetes who won't eat certain foods

My 10 year old nephew has a *STRICT* diet of Chicken nuggets/Sandwiches, Grits,Chips, Bannanas, Crackers and other snack foods and he has been like this all of his life; even as a baby he wouldn't eat baby food. He won't eat anything else, and when he tries to taste something different he gags and chokes as if the food is killing him. We have taken him to multiple doctors and they all looked at us like we were stupid, like we were lying about his eating habits. Then, just recently he was diagnosed with Type 1 Diabetes but has not changed. His sugar is high all of the time now and we keep telling him he has to eat veggies & fruits so he'll be healthy and not have to take the shots, but it's like talking to a wall. Is there any way we can try and make him eat other/healthy foods? Is there someone we can talk to about why his eating habits have been like this all his life? Please Help!