The young royal, seven, smiled and waved as he and his older siblings George, 11, and Charlotte, 10, arrived for the celebrations with their mother, the Princess of Wales.
The Prince and Princess of Wales and their three children have been spotted arriving at Buckingham Palace ahead of today's Trooping the Colour ceremony to mark the King's birthday.
Zak Kay met the royal couple in July 2016 aged 10, after he won a local contest to design a trophy for the Portsmouth leg of the America's Cup World Series.