It sounds like he just needs a little more saddle time. Spooking itself is okay in my book. Horses are hard wired to be afraid of things, its just a prey response. Running, bolting etc are not okay. When he spooks, make him stand there if you can. In essence, teach him to accept the thing he is afraid of. You can't train the spook out of a horse. You can train them how they respond to being afraid.
I use my voice as an almost constant source of reassurance (or annoyance!) for an anxious horse. Talk them through it. Sounds all hippy-dippy but it has always worked for me. Keep your voice calm and quiet..."Yes yes, that is a plastic bag...don't worry, it doesn't eat horses. Lets just keep moving on okay? Ohh look, it is a soda can, silly litter bugs leaving things laying around. No worries, it isn't hungry for horse today". By talking, I've found that I actually keep myself calmer, therefore calming the horse.