Since you son is very active with dance training, he must be burning lot of energy and have a high metabolism
. You may try the following:
1. Replace plain water with fresh fruit juice and milk
2. Feed him once every 1.5 to 2 hours, this may comprise of sandwich (cheese, eggs, chicken) and a fruit shake apart from the main meal of breakfast, lunch and dinner.
3. You may start adding clarified butter or ghee to his diet. About 1-2 teaspoon everyday instead of refined oils.
4. Feed him whole milk instead of skimmed, low fat.
5. Consuming badam milk (almond milk) twice a week is also very helpful. If he doesnt like it much then ask your son to eat about a handful of nuts like almonds, cashewnuts, pistachios everyday.
6. Add raisins and nuts to food and desserts.
7. Try including rich Indian preparations like: Butter chicken, paneer makhani, butter naan, rabri, jalebi, kheer to his diet as well. (optional).
These may be few tips but if adhered to will be helpful. Please share how its working for you.