Is it possible to leave at least one horse inside with him? That might be the best thing. One of mine pitches a fit and falls in it if she does not have someone in the next stall, one just doesn’t like to be in at all, and one doesn’t care as long as he has something to eat. Have tried several calming supplements, but none of them fixed that issue, although they did help with some others.
It is never the horse's fault