He was taken by his parents the Duke and Duchess of Cambridge to Buckingham Palace for lunch with most of the extended Windsor clan.
Joining the young family was nanny Jessie Webb, who sat in the back of the Range Rover as William drove through the palace gates.
On hand to coo over the baby were plenty of other youngsters, too.
One of William and Kate's bridesmaids, Lady Louise, 10, and her brother James, 6, were there, as was the newest royal family baby: Lord Freddie Windsor and his wife Sophie's daughter, Maud.
William and Kate's vehicle was the last to arrive for the lunch, observers tell PEOPLE. But the proud parents were all smiles when they left, with Kate waving to onlookers on the way home.
Missing from the festivities at the palace this year was Prince Harry, who is on his way back from the South Pole, where he and other teams successfully made an icy trek for charity last week.
The party is an annual affair for the entire family, with more than 40 people at the lunch. A smaller gathering will be held on Christmas Day at the Queen’s country home in Norfolk, Sandringham House.
William’s cousin Princess Beatrice, just back from New York (where she visited sister Eugenie), was driven to the palace by her father, Prince Andrew, while the pregnant Zara Phillips and her mother, Princess Anne, were also among the lunch crowd.