I was always taught in school that "he/him/his" was gender neutral if there was no context indicating otherwise. People have started to get more and more annoyed by this, leading to the grammatically incorrect usage of "them/they."
Anyone who went to school or learned English in 1980s or earlier would have had learned "Dear Sir" as the standard generic salutation when writing a professional letter.
I suppose today one would start such a letter with "Dear Person." That sounds somehow disrespectful to me. There's always "To whom it may concern."