He acts like the grand poobah of the office.
grand poobah
idiomA person who is important or high-ranking.
Origin
first recorded 19th c.From Pooh-Bah, the name of a character in Gilbert and Sullivan's comic opera The Mikado (1885).
The local grand poobah decided to close the park for the entire weekend without asking anyone.
Despite his lack of actual authority, he carried himself with the self-importance of a grand poobah during the board meeting.