Prince William is set to choose Pelly as one of six godparents for the newborn, in line with royal tradition.
The 31-year-old businessman, who organised William's stag party, could prove to be a controversial choice as he is currently facing trial over a drink driving charge, which he denies.
He has been a close confidant to both William and and his brother Prince Harry for many years.
A source tells Britain's The Sun, "He is thrilled to bits about being picked and everyone was congratulating him. He's been told informally by the prince. Guy's been a very discreet and good friend to William over the years. Those are qualities he wants to pass to George."
The other five close associates rumoured to be lined up as godparents are Prince Harry, Prince William's sister-in-law Pippa Middleton, and the royal couple's pals James Meade, Alicia Fox-Pitt and Thomas van Straubenzee.