Prince Harry, who is marrying Meghan Markle in Windsor on 19 May, was the best man when the Duke of Cambridge married Kate Middleton in 2011.
Prince Harry also joked with reporters stating that he had got down on one knee to ask his brother, who said he was "thrilled" to be asked.
Prince William said he was "working on" a name for his three-day old son.
Prince William and Catherine's third child, who was born on Monday weighing 8lb 7oz, is fifth in line to the throne.
