Maybe something horrible happened to him on a trail that he remembers? My suggestion is — love him for what he is and what he can do where he does it right and don’t force him to do something he hates — do what he likes to do and does well. If you don’t, all you will end up with is a miserable ride and a miserable horse and a miserable you. Leave the trails for a horse that truly enjoys being out. Your boy might just feel secure around the arena and the wide open spaces freaks him out and it sounds like nothing is gonna change that.