Try doing half raised poles walk through them first then trot them.this will help stretch him out alot and make him pick up his feet more. For me this has helped my horse jump better.
Start with trot poles and work up. Do elevated trot poles (where you raise them up a tiny bit and just trot regularly over them)
before trying to make the horse jump make sure it's comformation is up to par for it to jump. otherwise it can hurt itself
Pole work and smaller jumps. Basically go back to the basics and figure out where the problem lays.