He's out of office. を英語に訳すと？

「out of office」はもともと「外出中」、「出張中」、「休暇中」の状況ですが、今では「休暇中」という意味で使われることがほとんどです。

勤務中だがオフィスにはいない場合
He is not in his office.
He is working outside his office.
He is remote-working.
He is traveling.
He is on a business trip.
He is visiting (name).

休みの場合
He is out of office.

また、「out of office」は、公職員や政治家がその地位から退く意味もあります。

The scandal forced the governor to resign and step out of office immediately.