Weird. Most horses are better with other horses around and get silly when they're alone. Yours is the reverse. Anyway, the fact that he doesn't do it when you're hand walking him points to an attention issue on his part. If by chance he tends to be lower on the totem pole when it comes to other horses, maybe he feels like he has to always be looking to what they're doing and less likely to pay attention to you.
Is his spook big, or just a sudden side step and then he moves on? If it's the side step variety, the best thing you can do it pretend it never happen. If he stops getting a reaction from you, he might lose interest and develop a different interesting habit. (after all, don't they always have something new in store for us?)
If it is a bigger reaction, my first suggestion would be the drunken walk. Pick an area where you know you will come across something he will jump at and instead of walking dead straight toward it, use one rein and then the other to weave him back and forth toward it. Not a really big weave, but just enough that he has to respond to following your direction. It seems to transfer just enough attention to the rider that the horse stops worrying about the smaller stuff. You're already doing the right thing by making him face the object he spooks at. I wouldn't necessarily make him walk up to it unless he initiates the action, but if it works for him, do it. Once he conquers the smaller non-horse stuff, maybe he'll have developed enough reaction conditioning from you that he'll start to look to you before he spooks at other horses. Once most horses know they won't be let off the hook and be made to face the scary object, no matter what it is, many give up and learn to manage their fears instead of react to them. Good luck.
You just have to see your distance...you don't have to like it.