Jack may technically be a commoner, but his family does have some royal ties. They are descended from the Brooksbank baronets.
Sarah Ferguson, also happens to be the great-great-granddaugther of Lady Julia Coke, who is the daughter of Jack's great-great-grandfather Thomas William Coke, meaning Eugenie and Jack are third cousins once removed.
He has already attended several royal functions with Eugenie, the first being the Royal Ascot, back in 2011.