Because your son is already underweight I don't know that the traditional LC diet would be the best for him. However you may be able to modify it. Sugar for one is no good for anyone, let alone a child with ADD. I myself have ADD and found that this way of eating helps with concentration. Does your son have ADD or ADHD? I am no longer on medication, but I am an adult and body chemistry changes as you get older and with change of diet. You might also want to look into other carbs that don't give such an insulin spike. You can look these items up both in Atkins and the Zone books. I think the Zone goes more into glycemic index spikes. Using this method you would be able to keep his carbs up enough so he wouldn't loose weight and the insulin spikes down so he would be able to control himself and his thoughts better.

Hope this help,
Heather