Snezzy... I don't know how you took that situation, but I would've had 911 already dialed on my phone and my finger hovering over the call button... That'd scare the bejesus out of me if someone at my barn did that.
Anyways, Horses have amazing memories, they remember situations, and even training practices years after they happened, so this is on your side.
I would do some long reining in a surcingle, and see how he remembers it. Most horses come back to long reining easily. I would tie a lead rope through the top hole of the surshingle and then loop it under under his tail and then back through the hole in a cricle, because he needs to get re-use to the crupper. While you have this lead rope in place, long rein him like this and see how he does.
After he's doing well like this (walk-trot easily and not reacting) Put two stings on either side on a log and tie the strings long enough that if they were attached to the surcingle they would drag about five feet behind him, then put both ends of the strings in your right hand, lead the pony with your left (so your left hand is reaching across your body to lead him), place your hands with the stings attached to the log on his withers, and then walk him on. If he spooks at the log, you can let go immediately. This is just to see how he will react to the log. He will probably not spook if he's driven before.
After this you can actually tie the log behind him with very very thin string so if something happens it will break before major damage is cause (because you never know).
Anyways, once you can lead him around with the log behind him, try long reining him with it. The log should drag behind him about five feet at first (seems like a lot, but you want them to get use to it before putting it up to them, so they can see it, for a horse its usually nine feet). Once he's okay with that, move it up to where it's behind him about three feet, and then to two feet. This will get them use to something behind them.
This is all the training information I'm comfortable with giving you, because I don't want to mis-inform you. Good luck!