He was commonly referred to as Jack. My parents, who were fans, referred to him as Jack Kennedy in conversation around the dinner table and many news commentators over the years have used that nickname. Don't know for sure if the family used this name but I suspect that's where it originated.
My given name is John and while I've never been referred to as Jack (without the "wagon" or "ass" suffixes, my wife likes to use) I've known other Johns referred to by the nickname Jack.
Never trouble trouble til trouble troubles you. Fortune Cookie