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.
Moon in the title refers to Sun Myung Moon (Unification Church members are pejoratively called “Moonies”, referring to his name), so this word is capitalized as well (following RDA A.11).