Find out the reason behind of not gaining weight by doing concerned necessary tests and taking advice of a doctor.
If only healthy diet is suggested then, eat energy dense foods, Starchy carbohydrates like potatoes, colocasia ( Arbi) , sweet potatoes, grains, millets, nuts and seeds, milk and it's pdts, paneer, butter, desi ghee . Milk shakes, banana shake, fruits, chikoo, banana, avocado.
Next Steps
Know the reason