He wants to marry a Christian woman

Yes, you have to tell her family; in fact the marriage is not valid unless the marriage contract is done by her guardian or his deputy acting on his behalf. If her relatives refuse to let her get married, then guardianship passes to the Muslim judge who may do the marriage contact for her. 

The Standing Committee for Issuing Fatwas was asked: A Kitaabi (Jewish or Christian) woman wants to marry a Muslim. When her father, who is also a kitaabi, realized that his daughter might become Muslim after her marriage to a Muslim boy, he refused to be her guardian for marriage, and he refused to let her marry him. Please note that she has not entered Islam yet. Who should be her guardian in this case? 

They replied: The kitaabi woman should be given in marriage by her father. If she has no father or he refuses to do the marriage contract, then it should be done by the closest male relative on her father’s side. If there are no such relatives, or they refuse to do it, then her marriage contract should be done by the Muslim qaadi (judge). If there is no qaadi, then the head of the Islamic Centre in her area should do it, because the basic principle with regard to guardianship of women is that it should be the father, then the closest male relative on the father’s side, then the next closest. If there are no such relatives, or they are not qualified to be her guardians for any reason, or they refuse unlawfully, then guardianship passes to the ruler or his deputy. Allaah says (interpretation of the meaning): “The believers, men and women, are Awliyaa’ (helpers, supporters, friends, protectors) of one another” [al-Tawbah 9:71]. It was narrated that the Prophet (peace and blessings of Allaah be upon him) wanted to marry Umm Habeebah bint Abi Sufyaan who was Muslim, but Abu Sufyaan (her father) had not become Muslim, so the Prophet (peace and blessings of Allaah be upon him) appointed ‘Amr ibn Umayyah al-Damari to act as his representative in the marriage contract, and her cousin Khaalid ibn Sa’eed ibn al-‘Aas, who was Muslim, gave her to him in marriage.  

If the closest relatives of a free woman refuse to marry her to a man who is compatible and suitable, then more distant relatives may give her in marriage. If there are no such relatives, then the ruler should do it, because the Prophet (peace and blessings of Allaah be upon him) said: “The sultan is the wali (guardian) of one who has no wali.” 

And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote. 

Standing Committee for Academic Research and Issuing Fatwas 

Fataawa al-Lajnah al-Daa’imah (18/162). 

We ask Allaah to help you to do all that is good. 

And Allaah knows best.