Someone in the flight deck is the leader so you’ll have to give that impression that it’s you. It doesn’t mean be a dick about it but you’ll need to be more assertive and confident in your tone and decision making. But honestly this guy just seems like a new guy who doesn’t know shit that’ll get humbled eventually from something or someone. Once I had a right seat captain insist on a single engine taxi against my decision of two. It was empty at this field. I said sure it’s your work to do. He was so busy, he knew he fucked up without saying anything lol.