Hi, for low weight you need to first know that there are no other problems like digestion issues or malabsorption or hormonal imbalances are there or not. If all ok,then do have filling but nutritious meals at regular small gaps throughout the day.
Next Steps
Have wholesome meals n a filling heavy dinner. Drink milk and it's products, dal, sprouts rajma, chana. If non veg then eggs, fish or lean meat.
Health Tips
Contact with any dietician for personalized diet plan suited best in your lifestyle and body type.