Hi, Welcome to Health care magic forum.
As you describe, the causes for the low weight may be the genetic factor, some chronic disease, like intolerance to the milk,
tuberculosis etc.
I advise you to consult a physician for diagnosis and treatment.
I usually prescribe to my patient with such condition, injections of anabolic
steroids periodically,
protein supplements,enzymes, and multivitamins.
ask him to take more of green leafy vegetables, pulses, sprouts, and protein rich foods, to have good recovery, and resistance against infections.
Wishing for a quick and complete recovery. Thank you.