my son's wait is 11.5 it is very less compare to his age what can I do to improve his wait

A. To help the child gain weight, include fat and protein rich foods like potato, rice, pumpkin, sweet potato , chicken, fish, egg, legumes, banana, milkshakes with dry fruits, avocado, paneer and 550-650 ml milk everyday alongwith variety of fluids like water, fresh juices, curd, coconut water and buttermilk. Feed 3 main meals and 2-3 times healthy snacks.