Does the birth name matter that much? Isn’t a new name often given when they become king?
Queen Elizabeth’s father was King George VI but his birth name was Albert Frederick Arthur George. A mouthful. 
King George V birth name was George Frederick Ernest Albert
They always use one of their baptismal names, as the examples of Edward VIII and George VI show. Edward was known as David in the family, and George was known as Bertie, but their baptismal names included Edward and George, respectively.
I came in to say Ethelred Cnut, tradition and all that, but I see I am far from the first.
What is their last name?
One slip of the tongue by a nearsighted grade school teacher and the kid’s life is hell from then on. BEST CASE SCENARIO is they’ll just call him Ethel the Red Cnut, but sooner or later someone will switch the n and u and you’ll have a future king with a lot of people he wants dead.
Here’s a list of people who I’m going to shoot
Because life ain’t easy for a boy named Cnut
Their last name is Mountbatton-Windsor.
Are you sure its known to be a boy? There are those who still hope for a girl. He could be George Michael Elton John. He can go by the nickname Boy George. When he’s coronated he can be Freddie the 1st. What could be better?
Well. Officially, the surname “Mountbatten-Windsor” only applies to those members of the family that don’t have the style of HRH and the title of Prince/Princess. Their surname could be Mountbatten-Windsor, or it could be Windsor; in the military the princes have used “Wales” as a surname; and of course the new baby is Prince Something of Cambridge so he could also use “Cambridge”.
But you doesn’t have to call him Johnson.
[Older Dopers may get it.]
You can call me Ray,
or you can call me Jay,
etc.
They should name the baby Josey Wales.
I think WINSTON might be an interesting choice. Nice British name
That would be HRH Prince Philip James Henry George of Cambridge.
I like:
-Algernon
-Sholto
-Cnut
-Aelthestan
-or Clyde
Prince Diana North-West Kardashian Philip Artist Alfred Edward Henry Richard James George Marley Me Victor Victoria Hassenpfeffer Egregious Rondonaldrick Luther Mungo Muhammad Albus Severus Dakota Middleton Mountbatten-Windsor-Gonzales.
Aaron Nicholas
(that’s my IRL name)
Iggy for short.
Looks ginger - Hamish?
Iple of the Thing.