Maybe practice some leg yielding and maybe stand him in the corners. Mix it up :)
Try spurs and see how how does just don't jab him unless he gets naughty
i have the same problem with my pony but i use a crop for directional purposes but if you just tap them with it wont hurt them
hopr this helps
Even at the walk? If not, try starting him at the walk (in the corners), and if he responds well, trot etc..
perhaps add a little spur to your mix and whenever he ignores the leg completely remind him that he needs to be aware of it with a little tap of the spur
He bumps against the corners or just avoids them completely