You can do fun obstacle courses both within the arena and outside. Start in the arena in case he pulls away you can easily catch him. Use everything you have around the barn. Buckets, tarps, poles, feed bags, garbage bags tied to something so they blow in the wind, tires stacked up etc.
Let him walk by in and around them. Once he gets comfortable with that, move to an outdoor obstacle.
I have an excellent book called, Bombproof your horse by Rick Pelicano. It has everything you could ever want about de-spooking and how your horse reacts to different things. Here's the Amazon