[Jack is a sucker for anything World War Two, as Betty will see from the pseudo-period greatcoat he's wearing, so when he sees a hairstyle that's definitely 1940s (even if it really doesn't work with the jumpsuit) he can't resist saying hello. And saying hello Jack Harkness style means a blinding smile showing whiter than white teeth and a twinkle in his eye that can make grown men weak at the knees.