IMO, he's a little old to be just learning how to jump. Horses at that age tend to be set in their ways, not that they can't be retrained though. Usually if you work on collection and getting the horse to slow down, that is when they are working of their hind. The faster you go, the more they work off their front.
First rule out arthritis, soreness, out of alignment, etc. He'll probably need to see a chiropractor and possibly an equine masseuse. If he's seen a vet, chiropractor, and masseuse, and is still doing it (with no physical cause) then it is a training problem.
I just think he's too old to be asking him to do this. Jumping is one of the most physically demanding things you can ask a horse to do. Unless this horse is trained to jump, I would just drop it if I were you. To do any real or regular jumping, he is going to need a slew of supplements and health care, and that is if he is up to the job.