Are you saying that upon landing on the other side of the jump, his head goes reflexively down, almost to touching the ground? And does it feel like he's almost collapsing? If so , that would be a physical issue , for sure. I would think about the body and what parts are most stressed at the moment of impact , landing. Shoulder? I mean there's a lot going on whenthe horse absorbs all that momentum.
Just a thought . .. Sounds kind of dangerous to me.