As your son is still 15 year old, there is no need to worry about his height. He can grow naturally, as it is his growing period.
First of all I would like to explain how a height of a person increases:
A height of a person increases by a substance called Human Growth Hormone. It is released deep inside the brain (behind the eyes). This substance is released during the first two hours of sleep. This hormone works from childhood to maturity. Once the person is matured the hormone stops stimulating growth, and therefore he reaches the maximum height. Therefore plenty of sleep is needed for growth of body.
There are many artificial methods for increasing heights. But they are very dangerous as artificial chemicals are used and can cause allergic reactions. They are also very expensive and painful surgery.
Here are some natural methods which are safer to become taller:
a) Nutritious food:
Give him nutritious food such as, fish, meat, milk, vegetables, and fruits. Nutritious food will help him to increase the height. Food which is rich in vitamin, calcium, iron, protein is very good for growing. Milk contains lot of calcium, & helps in increasing height. Also eat food which is rich in vitamin D. Avoid fast or junk food.
Exercise helps to increase the height. Tell him to do some general exercise like running, jogging, swimming, cycling, hanging exercise, jumping, walking, stretching, etc. which will help him to become taller. He can also play games like volleyball, high jump, long jump & skipping regularly. Along with exercise he needs enough sleep. He can gain height through sleeping. He has to keep his mind calm and it can be done through yoga. As you have said that he is doing some asana, tell him to continue doing it. As yoga asana are very good.
Finally, I would like to advice you, not to go for any ayurvedic pills or any medicines. I don't think they are safe.