I reckon that you should by this poor old bugger and have him put out of his obvious misery! Alternatively, you could buy him and have him gelded. I hear squirrels love crunching nuts :-)
I admire your tenacity and your willingness to give the horse a chance but if you have excluded all of the obvious sources of pain then there is little else you can do. Often behaviour problems do arise when a horse has pain but once these responses are well established it is very difficult to...