What does it mean when your ex sees you and hangs his head and when you walk past he turns around and looks back at you?

Generally it's not comfortable seeing your ex. You may have caught him by surprise. He obviously doesn't know what to say given the fact you have split-up and is fearful of how you will react towards him.

Sometimes, the reason why you split up in the first place affects the way your ex will act around you. He could he ashamed -- of himself, for example, if he felt that it were his fault for your demise. Sometimes, it's simply awkward because they're your ex. If you're in high school, people will talk and that could play a role in why he does that. It'll pass over, so just don't worry about it.