Lots of excellent advice, but you don’t mention how much your horse is turned out or whether he’s with other horses. If you can just offer him lots of freedom in a herd situation (where the other horses aren’t afraid of their environment) I think he might work off a lot of this tension on his own. He can explore new sights, sounds and smells at his own pace. And if he is out with other horses, make sure another horse isn’t bullying him, especially if he tends to be lower in the pecking order. I’ve seen totally amazing changes in horses’ personalities by addressing just these two issues.