So, Nicholas's back completely gave out a couple of weeks ago, I / we were totally right about where the issue was coming from. (He has made a full recovery, nobody panic!
I finally convinced the owners to get a chiro out to adjust him, and they are going to start looking for a saddle that fits him better. Yay! In the mean time I am borrowing a therapeutic memory foam pad from a friend whose horse is prone to being backsore, which Nicholas really seems to like.
Thanks for all the help, everyone!