Hair thinning in your son could be caused by various factors, including malnutrition, poor constitution, or a deficiency in certain nutrients. It is recommended to enhance nutrition, exercise regularly, and ensure an adequate intake of protein, fats, and vitamins. Foods like eggs, milk, lean meat, fish, and soy products should be consumed more, and a moderate intake of cod liver oil and vitamins A, B1, B2 should be considered.