I'm fairly sure he will be George VII. He has four names so can choose which one he wants, like his grandfather did. He was Bertie (Albert) but chose to be George VI.
You raise an interesting point. Does the monarch have to choose one of their given names? A very quick google search reveals that all three (I would have guessed there had been more) monarchs who did not use their first given name when they came to the throne did use one of their given names, but I haven't found anything saying that they HAVE to. Could he be King Wayne if he wanted?