It is true a horse living outside will be way more calm for a walk. You can check your horse has not too much energizing food. Also, I know it is easy to say but DON'T PANIC! If you do, your horse will feel it and become more anxious. Breathe slowly, keep calm and act quickly and efficiently. You can talk to your horse to try to relax him and make him focus on your voice. Another advice, create a diversion, you will make the horse forget what he is worried about: go in the opposite direction, ask him to do simple exercises (bending, little circles...). Good luck!
Well a lot of horses are nervous or super excited outside as they live in a box. A horse living in a paddock or field will be much more calm when going for a walk. So if you can, let your horse in a paddock everyday for some hours, it will change his behaviour.
Indeed you should lunge him before you go outside. Try to don't get irritated, things would go worst as the horse would feel you're getting nervous. Also, try to know if he is highly strung because he is happy to go for a walk or anxious, so you can adapt your behaviour.