It's unusual, but a nod to the Mountbatten-Battenberg connection, I suppose. It's quite a nice name but I'm a bit disappointed that 'Philip' wasn't included.
It's therefore an indirect nod to Philip for Prince Louis of Battenberg, Philip's grandfather, was onetime First Sea Lord during WWI until he was hounded out of office because of xenophobia. Thanks to said phobia, the Battenbergs also became Mountbattens.
Maybe Philip didn't want his name used in front? Who knows. The kid has a name that isn't Arthur or Albert.
The surprise is the use of Charles!