William van Cutsem is one of William's best friends and their families have had strong ties since his childhood. The van Cutsem's rented Anmer Hall (now Prince William and Kate's country home) when William and his siblings were young, making them neighbours to the royals. They were so close that William's brother, Edward, was a pageboy in Prince Charles and Princess Diana's wedding and is the godson of Prince Charles. In turn, William is one of Prince George's godparents.
It's even been reported that as Prince Charles and Diana's marriage was falling apart, both Prince William and Prince Harry spent much of their time in the care of the van Cutsem family.