A source told the Sunday People that the 28-year-old royal has been telling everyone that the royal couple are having a boy and he is thrilled at the prospect of having a little nephew, the Mirror reported.
The source said that the whole family is excited about it and apparently Kate wanted a boy.
The insider said that the royal couple are working hard on baby names and think that they have it sorted but are not revealing anything to -anyone - not even Harry.
The insider asserted that the close inner circle all know that it is a boy and they are busily buying gifts with a boy theme.
Officially, the Duke and Duchess of Cambridge have refused to divulge the sex of their soon-to-be-born baby but seven-months-pregnant Kate dropped a couple of hints in March when she appeared to accidentally suggest that a girl was on the way.
It happened during a walkabout in Grimsby, when Kate was presented with a teddy by a local well-wisher and said "I will take that for my d.." - and then stopped herself.
Just a few days later she gave another clue - this time indicating that the baby could be a boy, when she revealed that she bought a Bugaboo pram in light blue.
According to sources, Prince William's cousin Princess Eugenie has also been telling her friends that the baby is a boy.