First, this boy may have sexual psychological disorder, and subconsciously always thinks he is a girl. Although he has not reached the level of transsexuality, he will feel comfortable using a girl's name.
Second, this boy may worship a goddess or experience an unforgettable feeling, and using a girl's name may be a commemoration or nostalgia.
Third, boys' unconventional idea is to be different from others, or perhaps to experience different feelings.