It isn't "Leo" it's one of his past lives. He could have died shortly after these events, and been reborn with enough time to age enough to become a soldier in WWII. After his death in WWII he becomes a whitelighter. He is shown all of his past lives, so his present self remembers that life. The "Leo" in the past, has no idea that his next life will end in WWII, or that he will become a whitelighter.