I would start by making a note of what he spooks at. Some very smart horses almost turn spooking into a game they're playing.
I have found that one way to curb it is to minimize your own reaction - do you tend to grab for the reins, make a lot of noise, or get unsteady when he spooks? Do you punish him for spooking? All of the above can make a spooking horse more likely to spook again, because you are adding your energy to the situation. He may view it as "Oh, the human is getting upset! I guess that means there was something to be afraid of!" Sometimes we add our energy by getting excited, upset, or scared, and make the situation worse.
Another thing is to make sure that you are working on your leadership on the ground. This doesn't have to be running him ragged in a round pen - just take him for walks in-hand, and see what gets him going. Keep control, but don't talk, control your energy, and work on being soft and focused.
If you don't know what "soft" feels like, try this: Put your hands on your belly, and take several deep breaths. Tense your belly muscles, hold for 4-5 seconds, and then release. Now, try to remember what that feeling feels like. Walk around your house and try to keep yourself "soft"- not holding tension. Practice! I know I tend to hold a lot of tension in my stomach without even realizing it, so this exercise really helps.
I like to walk a horse up to what scared them and start messing with it. Rustle grass, rock that scary trailer on it's wheels, kick a rock, sit on a boulder or stump for a bit and talk to him. Keep him aimed at whatever it is. Wait until he relaxes for even just a moment, then throw a party. Praise, scratch his withers, tell him he's the best horse ever, let him graze.
I also like teaching the head-down cue. I just worked on it Sunday with my new horse, who has a lot of nervous energy and is also herd-bound as heck right now. Once I got him into a working frame of mind, I used pressure/release to get him to drop his head (and the tempting little bits of grass growing in the arena didn't hurt, either...). A scared horse won't drop their head to graze, so when they lower their head, it's sort of a "reset" button. Once it's practiced enough, many horses will even start to use it to self-soothe in anxious moments.