You may call His Royal Highness...
"The Duke and Duchess of Cambridge are delighted to announce that they have named their son Louis Arthur Charles," Kensington Palace announced Friday. "The baby will be known as His Royal Highness Prince Louis of Cambridge."
As for the baby's official title, according to royal historian Marlene Koenig who spoke to Town & Country, the little one will be called His Royal Highness Prince Louis of the United Kingdom of Great Britain and Northern Ireland. However, like his siblings and famous parents, little Louis will go by His Royal Highness Prince Louis of Cambridge.
Either way, that's quite a moniker to learn how to write on a name tag at school!