Oh that one might plead for a man with God! The original here is obscure. It may mean, Oh that he (i.e. God himself) would plead for a man with God! i.e. would become a Mediator between himself and man, plead for him, undertake his defence, and obtain for him merciful consideration.
Or, nearly as in the Authorized Version, Oh that one might plead for man (i.e. mankind at large) with God! interest him on their behalf, and obtain a merciful judgment for them. The former rendering is to be preferred.
As a man pleadeth for his neighbour; literally, as a son of man (or, as the Son of man) pleadeth for his neighbour. If we take the simpler rendering, "as a son of man," then the meaning is simply, "Oh that God would plead for man with himself, as a man is wont to plead for his fellow-man!"
But if we prefer the other rendering, "as the Son of man," a Messianic interpretation will be necessary. (So Professor Lee and Dr. Stanley Leathes) But Messianic interpretations of passages that do not require them, and that have no such traditional interpretation, require extreme caution.