Sidepassing is so much clearer when a gate whacks them in the hiney if they don't :]
Lol that's a funny line, anyway it's totally true. It works a lot better also because we are so focused on not getting out leg taken off by the tree, we tend to work a little more affectivly with that motivation
Anyway back to the issue at hand
I have worked with herd bound horses before (like I said my mom's horse is a little herd bound himself, better now but still rears it's ugly head every now and then)
I have found that you get better resaults if the horse IS in a herd living situation. It may not be the easiest way to work but it does get better results in the long run because after we are finished training we generally put them into a herd. We then expect them to know how to behave when taken out of it to work. The problem with that is we never really trained them to know that everything is ok with this situation.
So, if it was me and my horse (this is just how I would deal with this situation and is only my opinion) I would start by having them in a herd with at least two other horses if possible. If not then work with what you can :) I would then take some time to strangthen his relationship with you. Once he is calm, cool, and collected away from his buddies (with you with him) then I would start working. This may take a day or it may take a week, go with what works for him.
Once he is feeling safe with you then start working a little bit in the arena on patterns, circles, figure 8, bending, side passes, anything he knows already. I would also start taking him out on trails (if he is capable of doing this by himself then that's the way I would start). When he is calm out and about at walk/trot/canter then I would introduce going for rides with another horse speradically, don't make it a thing that happens all the time or you loose some of the progress you have made.
Go into every session with him, be it a working session or a lets hang and eat some grass building a stronger relationship session, with possitive expectations. If we expect possitive outcomes then we are more likely to get it because A) we don't except bad behaviour as readily and B) they sence the possitive attitude coming from us and want to make us happy.
Hope this helps a little, if anything didn't make sense let me know and I'll try to word it differently. Good luck