First of all, a few minor corrections: your sentence should be “Kennst du Herrn Einseberg?" The verb is kennen (conoscere) not können (potere / sapere). “Herr” is in the N-declension, so the accusative is “Herrn.” Back to your question. You’re correct that “den” is a relative pronoun and “ihn” is a personal pronoun. In German, especially colloquial German, it’s not unusual to use the relative pronoun instead of the personal pronoun, at the beginning of a sentence. This can give the word added emphasis, or simply make the phrase sound more casual.