Rather than focusing on calories and grams of fat, I think it is reasonable to focus instead on getting the appropriate servings of each particular food group. Your son needs to learn which foods he can eat as much of as he wants, and which foods he should avoid or eat rarely. This is a gradual process that takes time, and commitment from the family. The whole family has to adopt this healthful diet, not just your son. Additionally, if the whole family is doing this at home, it makes it a little easier for your son to continue a healthful pattern when he is away from home.
It would be helpful for you and your son to meet with a nutritionist who can go over basic dietary guidelines. The nutritionist can also give you an estimate of calories that your son should take, based on his level of activity and current rate of growth. There are also a number of children's groups available that specifically focus on children ages 8 to 15, and provide nutritional advice and counseling. You should speak to your doctor or a nutritionist.