Hi, Welcome to Health care magic forum.
The reasons for the under weight may be the genetic factor, or some chronic disease, or some problem with the intestine like intolerance to the milk and its products, wheat, barley etc.
I advise you to consult a physician for diagnosis and treatment .
I usually prescribe to my patient with such symptoms injection of
anabolic steroid periodically, enzymes, multivitamins, and
protein supplements.
Encourage him to take, more of calories, fats, fries, junk foods, ice creams etc.Also let him take green leafy vegetables, pulses, sprouts, and protein rich foods.
Wishing for a quick and complete recovery. Thank you.