"The first big thing that is going to happen to him is the arrival of a new baby brother or sister which will almost certainly be next year," Thornton tells PEOPLE.
"The advent of a new sibling will make a big difference to George because he will be knocked off his pedestal, but already we can see he is a charming, outgoing, confident little person."
The rumor mill is already on overdrive about Prince William and Kate's plans to expand their family. Recently, British bookmakers stopped taking wagers on a pregnancy announcement, fueling speculation. The Palace has not commented on the couple's family plans, but a friend has confirmed to PEOPLE that they want another baby in the not-too-distant future.
Thornton has been surprised by George's outgoing personality, most recently displayed during the royal family's April tour Down Under. "As a Sun Cancer with Moon in Capricorn and Scorpio rising, I expected him to be strong and fixed in purpose," she says. "But I didn't expect to see him so comfortable in the public eye. I think it shows how well Kate and William have brought him up."
So can we expect a burst of temper tantrums as George heads towards the "terrible twos"? "I expect he will continue to put his little foot down," Thornton tells PEOPLE. "But he is a curious, happy little person who is the epicenter of his parent's love and attention."
Other adventures could be in store for George in the next year, Thornton says. "Jupiter is now in a new place, so maybe some more travel," says Thornton. He may need the getaway. "His first big challenge will appear when baby number two arrives," Thornton says. "And that isn't so very far away."
For more on Prince George's first year, pick up the new issue of PEOPLE, on newsstands now