Leita Lawrence:When he rides with a longer rein, he gets left behind over the fence because he will not ride into a forward seat.
Definitely sounds like he's using the reins to balance AND overdoing the contact. However, if he can't maintain a forward seat (two-point?), he shouldn't be jumping. Although he has the experience riding, he might not have the tools for proper equitation and balance to be able to use his seat and legs effectively and maintain a two-point, without balancing on the horse's mouth. Riding with a shorter rein should NOT cause him to hang on the horse's face if he's properly balanced.
As has been suggested, lunge line lessons would be a great start. He needs to relearn balance and how to use his seat and legs for cues. Have him practice his two point, with and without stirrups. While he's in the two-point, have him move his hands up the neck to simulate the crest release and ride that way for a few strides. Can he do it? Also, check stirrup length. He might be unable to balance himself if his stirrups are too short.
When he's riding on his own, have him practice transitions within gaits without relying on his reins. That will help him get used to using his seat and legs and not rely on his reins. Practice proper downward transitions for the same reason.
Good luck and I hope you can help him!