I think the two pieces of advice I'd give would be:
1. Make sure that when you cook, you carb count so that you know exactly how many carbs there are in a portion of food. This will make management for his parents a lot easier.
2. Don't overuse sugar and try to use wholemeal flours, pastas, rices, etc, as these tend to have a slower release than the white versions.
Many might advocate a low carb approach to cooking for a newly diagnosed diabetic child, and while this is possible, I'd suggest that that is a decision that you need to leave to the parents. In the meantime, making it as easy as possible to work out the amount of carbs in what is being made and trying to avoid using excess carbs is probably the easiest approach to take.