Make sure his hip is down and his hind end is reaching well underneath himself. Sink just a tad more weight ino your outside seat bone, to help balance him around the corner. Be sure your shoulders are up and square, and your lower back is soft. Lifting the inside rein just a hair can also help balance the horse around turns.
If that doesn't help, you might want his hocks and stifles x-rayed.
"To be an equestrian in the classical sense is not just to be a rider. It is a position in life." --Charles de Kunffy
Anti-Slaughter and Proud!