There are other factors involved too--how much fat that is
in a food can make a difference too.
French fries are digested differently than say a baked potato.
Now, that may sound scary, but it doesn't have to be difficult. Try to make sure you are balancing carbohydrates and proteins with every meal. Make sure he gets a well rounded meal that is in good portion control. Also if he eats breakfast, snack, lunch, snack, dinner, snack--smaller meals all day may be very helpful as well.