Please understand I do not have experience with this problem, but in more than one book I've read that wide jumps, like a staircase--three ascending verticals, give the horse time to bring his legs up. From what little you say I would probably want to do the wide, stepped jump with your horse jumping loose or on the lunge line at first (be sure to run forward FAST if he is the lunge).
Have you checked for knee, elbow or shoulder pain? If he is an OTTB he might have a hidden problem like a small bone chip that is interfering with him bending his knees.
The only other thing I can think of is to get him thinking about his front feet, maybe putting protective boots or bell boots on or taking them off or switching back and forth.
In my experience, the best exercises for helping a horse improve his jumping style (and therefore efficiency) is with gymnastics. Gymnastic jumping is a line of multiple jumps with different distances between them. It requires the horse to balance himself, gather himself, and carry himself between the fences. Jumping is 95% flatwork, so by improving your horse's journey to each fence, you'll automatically improve every jump!
Here are some good examples of different gymnastic set-ups and exercises: