My seventeen year old daughter has Type One Diabetes. She was eleven when she was diagnosed and there was very little warning that something was wrong. After the diagnosis I did remember noticing that she was drinking an awful lot of water, but she was every athletic, active and it was summer. If your child is indeed diabetic and it seems like that may be the case, you are lucky to have a warning. We didn't have a catastrophic occurrence and you won't either. One of the most annoying policies that our "modern" endocrinologists employ, is the "you can eat anything" policy and just balance it with more insulin. My daughter is slowly moving more and more towards a lower carb diet with very good success. I don't think that the VLC diets that many employ here is a great idea for a young person, diabetic or not. But something in the "Zone" ratios seems easy, healthy and effective.
Hopefully your child is not diabetic and the high A1C is due to some other factor. Good luck and take care.
Some of you may die, but that is a risk I'm willing to take.