Welcome to HCM & thanks for writing in.
The condition is called canities.
Its mostly genetic and partly environmental.Some nutritional deficiency
is also associated with hair becoming grey.
Biotin ,calcium pantothenate
and N acetyl cysteine
intake as hair supplement will stop further progression of hair going grey.there is no proven medication for turning white hair into black.
Consultation with dermatologist
will allow prescription medications.
Hope this helps.
I wish him good health.