Working him up and down hills will help him. If you go up and down hills, try doing it side ways for a while. Start at the bottom of the hill and go diagonally across. at the same time start to move slowly upward on a diagonal line. Go back and forth until you reach the top. Do the same thing going down the hill (kind of like a beginner snow skier traverses down a medium size hill). This really helped my horse. He had the same problem.