Why has he been chosen as a leader?
Are the other players obeying his orders even when they don't make a lot of sense? Why?
You don't have to accept this character as a leader just because he says so and starts commanding you.
Maybe you should make your decissions as a group instead of relying on a single player to take the leadership role.
While this may make for an interesting short story, it will drive players away in a short time.
To me this sounds like a player issue, not a character issue.
Explain to the new player that others are not enjoying the game, so changes need to be made to make the game fun for others as well.
Perhaps in game the group could find out his character was being controlled by an enchanter or a demon. Once freed, he could step back from leadership while he processes what happen. A new leader could step to the front and take the character under his wings encouraging him to learn what good leadership is.
If the player is not willing to do this, perhaps he could make a new character.
In games I play leadership shifts with what is happening in the story. We have combat leaders, planning leaders and social leaders. If one player does all of this it steals the fun from other players. Talk about sharing leadership this way with the group.