If you have ruled out physical pain and tack fit issues, my suggestion is to go back to the basics. It could be the pony not wanting to jump, afraid to jump or not feeling balanced or it could be you interfering in some manner, dropping him ahead of the fence, off balance, popping him in the mouth when he does actually jump.
You said he is an amazing jumper and clears pony height fences easily but he refuses most of his jumps so something there is confusing me. He may be able to clear a jump but it is clear he doens't have the attitude of a willing jumper.
Basics: let's address you first...do a lot of two point at the trot and trotting over ground poles. Make sure your position is really strong and steady. Knot your reins at about the withers and go through the poles, in two-point, with your arms outstretched...you may find your two-point isn't as solid as you think and will give you a place to start to strengthen..it isn't never a bad idea to go back to this type of exercise even if your position is very solid. You have to remember that YOUR balance affects the horse's ability to jump. If a horse is confident and competent in jumping they can handle a little rider unbalance but if a less experienced jumper the horse needs the rider to be better balanced and consistent in that balance.
The pony: Before actually jumping this pony again, my suggestion is to lunge him over jumps first. If he jumps more easily without a rider than it is a balance issue, either his with a rider or your balance itself. If you have the area available, allowing him to free jump (not attached to a lunge line and no rider) may also help. Once he seems more confident, start at the bottom again when you ride...do poles, then small cavaletti, then small jumps etc. You need to rebuild his confidence in jumping. Trot and canter over poles...pay attention to your distances and your position.
As with others, if you still can't stop the refusals a trainer is going to be a must. We can give suggestions here but since we can't see what is happening we can only guess as to the actual problem. Yelling, kicking, using a crop before a jump won't solve the problem..it only makes the horse/pony more afraid of the rider than they are of the jump and that is a major accident just waiting to happen as they are highly unpredictable as to their reaction to a jump. If this was an occasional issue the voice or crop reminder may be just that, a reminder; but your issue sounds much deeper than just the occasional slip up.
Last edited by tlkng1; 11-03-2013 at 09:03 AM.