Gymnastics would help, because the bounce encourages the horse to have to tuck his legs over the jumps.
Try doing a lot of gymnastics and grids.... it may be messy and uncoordinated at the start but after a while your horse should begin to tuck in his legs and use his body.
Depends on his age. If young then it will take time. Gymnastics, be giving with your hands so that he jumps though his back. And then more gymnastics. Religiously.