What does it mean , when an underweight person at the same height as a healthy individual has small bone like hands and feet does he have low bone mas?

Not at all: Most likely the size of the hands and feet are genetic so you could ask your parents/grandparents about others in the family with similar sized hands/feet.

May be: a normal small person. If accompanied with any other symptoms consider evaluation for genetic abnormalities. If normal genetics consider weight lifting under supervision and exercise to improve muscle bulk.

Probably, NotCertain: Given limited information & the complexity of human body anatomy, this assessment likely has some accuracy. Since body anatomy, over time, usually adapts to body use & build in a effort to enable survival, low body weight/nutrition/demands for carrying body mass does not drive high bone mass. At the same time, external features do not always reflect internal structure for each individual. Why?