There are generally three possibilities for this situation.
Firstly, this boy may have a sexual disorder and subconsciously thinks of himself as a girl. Although he has not reached the level of transsexuality, he feels very comfortable using a girl's name.
The second is that the boy may worship a certain goddess or have experienced an unforgettable relationship. Using a girl's name may be a kind of commemoration or nostalgia.
The third is this boy’s unconventional idea, just to be different from others, perhaps to experience a different feeling.