And now his mother, the Princess of Wales, has made a funny confession about her youngest, who will turn five next month. Take a look at some of Louis's very best moments here...
The mother-of-three was speaking during a visit to a therapy garden at a rehabilitation centre in Wales when talk turned to her family.
She revealed that Louis had a special request every day after he finishes school; Louis and his siblings, Princess Charlotte and Prince George, all attend Lambrook School in Bracknell, Berkshire.
Kate revealed that little Louis feels the need to let off some steam after a day of lessons by spending time outside. She said, "Louis comes homes and says, 'I need to get my energy out!'"
Lambrook is the first school that the royal children have been able to attend together. They enrolled there last September – take a look at their first day at school in the clip below.
George, Charlotte are Louis now in Year 5, Year 3 and the reception class respectively at the prep school.
While George, Charlotte and Louis are three of the most famous children around the world, they're not referred to by their royal titles at school.
The siblings are known simply as George Wales, Charlotte Wales and Louis Wales to their school friends, in a nod to their parents' new titles, the Prince and Princess of Wales, which they were given following Queen Elizabeth II's death last September.
This was also the case for Prince William and Prince Harry, who took on their father Charles's former title Wales as their surname at school. Members of the royal family traditionally don't use a surname; they are simply known by their first name in the public eye and His or Her Royal Highness.
They can also be known by the name of their house, such as Windsor, which may be different to their surname, such as Mountbatten-Windsor.
LISTEN: King Charles' adventures from the cockpit while piloting planes
Make sure you never miss a ROYAL story! Sign up to The Royal Explainer newsletter to receive your weekly dose of royal features and other exclusive content straight to your inbox.