Prince Louis, four months, was last spotted in public during his christening service in July, with another appearance not expected to be until Christmas.
His older siblings, Prince George, five, and Princess Charlotte, three, join their parents on public outings more often.
They are close to their grandparents, Prince Charles and Camilla, Duchess of Cornwall, after an adorable video of the younger royal waving them goodbye emerged.
However, they have a very special nickname for Camilla that Prince Louis will most likely copy.
Prince George and Princess Charlotte call Camilla “GaGa” instead of Grandma.
The 71-year-old previously told the Daily Mail in 2013: “My own grandchildren call me GaGa.
“I don’t know if it’s because they think I am! It is funny but is still very sweet.”
To call someone “gaga” often means crazy or foolish, something that Camilla was insinuating.
She even told Lady Gaga of their similar name at the 2016 Royal Variety Performance.
While she didn’t explicitly state George and Charlotte called her this, it is likely the moniker she takes from the youngest in the Royal Family.
Prince Louis is expected to copy his older siblings and also call her Gaga, something much easier for younger children to say.
Camilla Duchess of Cornwall currently has eight grandchildren, including Prince George, Princess Charlotte, Prince Louis from Kate and William.
Camilla is also grandmother to Eliza Lopes, Louis Lopes, Gus Lopes, from her daughter Laura Lopes.
Lola Parker Bowles and Freddy Parker Bowles are her grandchildren from her son, Tom Parker Bowles.
Similarly, Prince George once called the Queen “Gan-Gan”, as revealed by Kate.
His father, Prince William, chose to call the Queen “Gary” as a child.
Gossip reporter Richard Kay told the Daily Mail how William confused household staff after calling for Gary, only for the Queen to confirm it was her.
Other royal nicknames include “Wombat”, which Diana called Prince William, and “Poppet”, which Prince William called Kate Middleton.
Meghan and Harry recently revealed theirs for each other, with Harry calling her Meg, while she calls her husband Haz.