The best approach is to redirect your horse's attention by changing his focus. Direct him to move his feet. Turn him to the right, walk him forward, turn him to the left, urge him to take a few steps backward. The instant your horse starts to relax, pet him and tell him what a good boy he is. This way you reward his bravery, not his spooking.
For more information, please read the story How to De-Spook a Spooked Horse.