Well, be patient and try not to become obsessed... He will reply if he wants to go out, and not if he is either busy or doesn't want to go out. If he doesn't reply you could always try texting him in a week, asking again. But let there be some time in between your messages, or you'll seem desperate, which might just push him away. In short... don't spam him with texts if he doesn't reply, and try to think of something else to pass your time with. Easier said than done, I know.