Hi again Rebecca,
When your horse spooked keep working him through it. Get his mind back on you by doing leg yields, collecting the trot/canter, lengthening the trot/canter, and transitions. Also put some ear plugs in before you ride to see if that lessens the spook.
If he spooks just keep him working, don't let him stop as if he learns you'll let him stop when he spooks he'll use it as a trick. Just tell him it's okay and reassure him while continuing on, my mare would 'spook' at a flat pole in the arena which she's passed a million times to see if it would work on me, eventually she figured out I wasn't easily fooled
I have an OTTB who used to spook a lot. I did lots of liberty work in the paddock and in the round pen to gain his trust, then I gradually introduced the "scary" things to him and eventually he got used to it. If he spooks while you're riding always remember to keep calm and patiently try again and again to get him to walk near or past whatever he's spooking at. When that doesn't work I usually dismount and lead him to or across whatever he's spooking at.
When desensitizing, have him in a round pen or small arena, take bags(or any object to desensitize) and just move it around, let him sniff it. Before shaking it have a grip on the lead rope, if you choose to have one. I don't really desensitize with a lead rope anymore, if the horse chooses to run away, he or she won't go far and I always go back to them. It's how I taught my buckskin mare.