It sounds like he could benefit from a calming supplement. It won’t make him mentally slow, but it will help take the edge off. Also, since he is not a easy keeper, have you considered cutting back a bit on the grain and adding beet pulp to his grain? I boarded at a place in TX and saw this work with a large warmblood with some gastro issues. The barn owner swore by it.
Hope this helps. Good luck with your guy.