I really hate spooks! I used to ride a very spooky pony and I had to work hard to keep it from getting to me!
I'd say first of all to watch what you're doing when he goes by and make sure that you aren't getting tense or riding defensively at all. (i know, soooo easy to do, right?) Remember that you are their herd leader and if you get apprehensive at all knowing that he'll probably spook, there's a good chance he's going think you're afraid of the monster too. If you don't feel like you can go by it relaxed, you could try just lunging him past it so he'll have to go by it again and again and hopefully after time realise that it's not a horse eating monster. If that's too much then start off walking as close as you can and feed him some treats as he gets closer. Some horses are braver with you on the ground, but they do need to learn how to be equally brave with you in the saddle.
Also, never get off when your horse is misbehaving unless you feel like your safety is in danger. Horses learn very quickly how to get out of work, even if it's just for a minute or two. I've seen some that have learned to spook at things to intimidate their riders and make them get off! I've also seen some horses spook out of boredom, which can be cured by making them work harder!
I'd start off by walking by at a bit of a distance with him bent away from the area and leg yield in a step or two walking past it. Get closer and closer. Make him work and pay attention to, doing things like shoulder-ins if you know how. Once he walks past it, then reward him with a pat. Some people talk soothingly to their horse and pat them while they're spooking at it, I guess to 'calm them down'? But horses can read that as a reward for spooking at it! So reward after. Be firm during.
Also... I JUST read this fantastic article in Practical Horseman about a horse with vision problems who started to spook. They get these cysts in their pupils that make it hard to see, especially when they come into sunlight. So make sure it's not a vision problem as well!
Hope that helps a bit! Good luck!