All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The questioner did not mention what will happen if the family of this person knew about his Islam to determine whether he is forced to break his fast or not. In principle, this person must fast as long as he meets the conditions of the obligation of fasting which are: (the ability to) reason, maturity, ability and being a non-traveler. It is impermissible for him to refrain from fasting merely to conceal his Islam from his family. The Muslims should help and support such a person as much as possible, materially and spiritually, to remain firm on his religion and adhere to the teachings of Islam.
Allaah Knows best.