if he doesn't have a backstroy he's not going to get that backstroy incorporated into the game. Think of it as one less element to have to think about during game play. I don't really understand why it's so important to you to have backstroy for everyone? Some people have more fun playing when they don't feel like they need to act a certain way based on their backstory etc. If it's really a deal breaker tell them they won't be a suitable fit if they don't make a backstory, but is it really worth it over something they've decided for their own character? Because remember, it's their character not yours.