as you are thin and lean, you take well nourished diet having more protein, moderate fat ,carbohydrate
in your diet.you can take milk with high fat.
you can add dates (khajur),carrots,gaud in your diet.
you can take some good branded protein power with milk
pl. do not go for any steroids
or anabolic medicines.
as you are very thin and lean, please rule out any disease underlying