I wil, give you some advice as a nurse and I'm sure one of the frequent DIS posters with diabetes or whose children have diabetes will chime in.
If the child is not your child, it would be important to find out what the parents are doing in terms of foods. He may not need anything special, but could eat the regular desserts.
All restaurants do have some 'no sugar added' desserts if they want to do that. The no sugar added desserts tend to be fruit or no sugar added brownies.
If you will be at a full service restaurant, they could probably provide some other substitute with advance notice.
Most diabetics are no longer taught to avoid sugar.
In the 'way past', many people with diabetes ate sugar free items and avoided anything with sugar. But, many of the sugar free things have as many 'carbs' (carbohydrates), which the body breaks down into glucose. Many also have more fat or more calories than the low carb versions of the sme food.
And, many things like fruits have natural sugar.
Here's a good reference:
http://www.mayoclinic.com/health/diabetes-diet/DA00072
Many people have issues with some of the artificial sweetners - they can cause upset stomach or even diarrhea.
Most diabetics now are taught carb counting and adjusting their insulin based on the carbs they are eating and the combination of foods they are eating (eating sweets with a complex meal slows the rise in blood sugar) rather thn trying to eat sugar free.
This doesn't mean they gorge on sweets, but do it in moderation, as part of a generally well balanced diet.
Studies have shown that people do better if they are allowed everything in moderation and don't cut out classes of foods.