Speaking to schoolchildren at BBC's Bridge House in Media City in Salford, the Prince said his son's performance at Thomas's London Day School in Battersea was a 'funny' one.
Asking the children about their plans for Christmas, William said: 'I went to my boy's nativity play. It was funny.'
He added: 'He was a sheep.'
The prince and wife Kate, the Duchess of Cambridge, visited the group during a 'stepping out' session, in which the children gave their feedback on new programmes. The couple also encouraged the children to talk about their mental health.
The royals are spearheading a Heads Together campaign, which is working with charities with the aim to end the stigma around talking about mental health issues.
'People are so worried about what they say, they aren't
saying anything at all, and what we've found from speaking to people is
it's so good to have conversations,' said William.
'Mental health used to be this scary word that people didn't like talking about, and it's slowly getting better now,' added the Duchess of Cambridge.
When Prince William last spoke of his son's school days, he remarked that George had quickly grown fed up with the daily school routine.
The royal said that it took a while for his son to realise that he would go to school every day.
'I just dropped George off and he didn't want to go,' the royal told a fellow parent on a visit to Milton Keynes.
Alongside sister Princess Charlotte, George will soon be joined by another member of the family, as the Duchess of Cambridge is expected to give birth around March 2018.
George and Charlotte are also set to play a part in uncle Prince Harry's wedding when he ties the knot with actress Meghan Markle next spring.