Option A: 'He' refers to the boy. 'He' is a pronoun that represents a noun that has been mentioned before. The nouns "his son" and "the boy" have been mentioned before, so 'he' refers to the boy. Hence this option is most appropriate, and option A is correct.
Option B: 'They' is a pronoun in the third person plural. It refers to many people, whereas the pronoun must refer to only one person, the boy had written about "his father".
Option C: 'She' refers to a girl whereas the sentence is about "the boy".
Option D: 'We' is a plural pronoun and refers to people more than one. The boy is singular.
Hence these options are wrong.