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.