My son's children are small and healthy. Their doctor says the same thing. Using that damned scale! Our family is all small framed, and no one is overweight. We all eat good food, and that is just how we are all built. He will be fine as long as he is eating good food. Some are thin, some are medium, and some are heavy. If you worry too much about his eating, you may start a battle with him later on. He will be fine. You sound very caring.