Unless there has been a trauma to the back - horses RARELY have back problems which are not symptomatic of other problems. Just like in humans how back problems are usually postural, caused by poor shoes or knee or ankle problems unless the person has fallen, been in a car accident, etc..
So. By what you've said I would look at the horse's joints. Older horses, especially ones who are jumped or otherwise worked hard are very prone to joint issues. I would first of all suspect the hocks as one of the major symptoms that vets use to diagnose hock issues is lower back pain. It would make sense that the pony is launching himself forward, not up, to avoid loading the hocks.
Call the vet.
I've actually ridden a mare with major vision issues. When she missed fences she crashed through them like they weren't there. She didn't launch herself into a long spot as this pony is doing. Keep in mind that about 3 to 4 strides out from the jump, the horses lose sight of it completely as they cannot see directly in front of them.
They say money doesn't buy happiness -- well happiness doesn't buy horses!