Someone with undetermined/unknowable gender would use the pronouns they/them, never he/him.

We’re not discussing what someone would use for themselves. We are discussing what someone would use when writing about a hypothetical person.

If you believe that he or him would never be used in this case, then I suggest you do some research on the history of language.

Edit to clarify: And by history, I include recent history, meaning usage by people alive today, who learned it in school not terribly long ago.