For some reason (probably advertising), the word "relax" does seem to be pretty wide spread. It's made it into European Spanish, for example, where I have even heard it used as a noun and an adjective. But Americans also use it fairly often in daily speech - "Just relax!" for example, when we want to calm things down but don't want to yell "siddown and shaddup!" So it's possible he heard it used by our troops.
Ok maybe you are right about "relax" but he was out "picnicing" in a war zone?