I think it's to do with the missing two years of Jack's memory. It's probable that this guy is either another conman and has stolen Jack's identity, or Jack stole
his identity... or something like that.
As for the rank insignia he's wearing that could get confusing because US Army Air Corps ranks and RAF ranks are different, and the equivalent in the RAF is Flight Lieutenant so whatever.... we dont even see the guy's rank insignia in the preview anyway, because they would be around his tunic cuff's so why am I even going on about it.