He doesn't have to leave. He just can't be a mayor unless he's willing to apply to German citizenship (which he'd get in a heatbeat) because you're required to be an EU citizen for that. Have you read TFA at all?
> Please don't insinuate that someone hasn't read an article. "Did you even read the article? It mentions that" can be shortened to "The article mentions that."
Also, from the guidelines:
> Please don't insinuate that someone hasn't read an article. "Did you even read the article? It mentions that" can be shortened to "The article mentions that."