I understand your concern. Check your family history of the stature n built at this age, your hormone levels, any digestive disorders or worm issues if any with a physician.
If all OK, you can try to have a calorie rich n nutritious foods like fruit smoothies, milk shakes, fruit custards, nuts,wholesome cereals like oats, Dalia, roti or rice. Have proteins from Chana, soyamilk, dairy products, if non veg then have eggs, fish n lean meat. Try to have a heavy dinner, which aids in weight gain. Do few cardio m strengthening exercises.