The Moon is not the Son : a close look at the teachings of Rev. Sun Myung Moon and the Unification Church / by James Bjornstad. (OCLC #2985407)
RDA A.17.1 on Capitalization of Deities says to “capitalize the name of a deity and any term referring to the Christian Trinity”, and includes as examples:
Son of God Son of Man
That seems to be the “Son” referenced in this book’s title, so it is capitalized there.