He's walking ahead of you and head butting you? I don't care how "nervous" he is that is disrespectful! He might be less nervous and more secure with boundaries. If his previous owner was afraid of him, he may be used to running all over people. This needs to stop. It's sometimes amazing how horses can calm down when they know their limitations; when someone else is in charge. Horses are a lot like kids. They like knowing someone sets limits for them: it helps make their world predicatable. A nervous horse needs a predicatable world, it gives him less to worry about. So don't excuse his behavior because he's nervous. That's just another reason to make sure he behaves!