There is only one in the whole world

[muriel_volestrangler]

3. As far as I know, she is not yet a British citizen

This (from when they were engaged) says she was going to go through the normal process, which takes years, and I haven't heard anything saying that's changed for her: https://www.bbc.co.uk/news/uk-42169602

So the boy seems to qualify under:

A person born on or after November 14, 1986, is a U.S. citizen if all of the following are true:[16]

The person's parents were married at time of birth
One of the person's parents was a U.S. citizen when the person in question was born
The citizen parent lived at least five years in the United States before the child's birth
A minimum of two of these five years in the United States were after the citizen parent's 14th birthday.

https://en.wikipedia.org/wiki/United_States_nationality_law#Birth_abroad_to_one_United_States_citizen

However, I'd think this would be a problem for anyone trying to be king and president at the same time:

"No Title of Nobility shall be granted by the United States: And no Person holding any Office of Profit or Trust under them, shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince, or foreign State"