This seems like it was a taught behavior. Have you talked to the owner?
Does he do the same on the lunge or free jumping?
You can also put a 2nd jump, a stride after.
When he stops stay calm and ask him (progressively more) to go on after the jump.
This would be a great place to use treats.. Place a barrel or something strait ahead, on which you can put a treat.. he will "very" soon learn that you want him to go on after the jump, or have someone there (even better as the person can move to different places.. once he has done it well a few times at the same place)..