He'd done such things for decades, as in "Jack, I'm ready! Look at me now!" This was no blip in a much-married man but a usage akin to Joya Sherril's 1945 singing with Duke Ellington: "Kiss me long, strong and consecutive, no short snog will suit me, Jack . . . when I go fishin', a minnow won't do."
It wasn't the name but the lips that counted. Despite the movie I'm All Right Jack, the word has fallen from use. The history of this familiar, general term is uncertain but long: a variant of John, it echoes Jacques, which in France was also a way in which one might address a peasant without troubling to ask his real name.