How do I make him stop?

Watching my nephew for 2 weeks well my sister and her husband go on a trip. 2 days into staying with me, he said I look like Mr. Beast and that I should tell people I'm Mr. Beast when we go places so they think I'm famous. I said that I won't do that because it won't work and it's wrong to impersonate other people for your benefit. However, he just started doing it everywhere we go.
>Go to the park
He tells all the other kids I'm Mr. Beast.
>Go to the store
He tells the cashier I'm Mr. Beast.
>Go to the Ice Cream Shop
He tells everyone in line I'm Mr. Beast.
I've told him to stop. I've tried to explain why he shouldn't do that. I even tried going along with it today to see if he was just doing it to make me angry, but then he was just happy I was going along with it.

How the fuck do I get him to stop? My sister isn't back till next Saturday. I can't do this for another week.