The tall stature of your brother is familial or constitutional, and not due to any hormone imbalances. Still it will be prudent to consult a paediatric endocrinologist nearby to evaluate him and screen for any hormonal excess. Very rarely, this could be due to excess growth hormone secreted by the pituitary gland. It is possible to arrest further growth and height by hormone therapy. Teach him to make the best use out of his height by playing games like basketball, tennis etc.