I suspect the issue is that he has a lot of thrust and really rounds his back over the fence, not that he pulls to the base. The thrust and rounding of the back is pushing the rider forward onto the horse's neck.
This is pretty common when you move from an equitation or suitable for an amateur type horse with a flat jump to a horse that really uses itself correctly, and it will expose any fundamental flaws in your position. Riders that pose or perch probably couldn't ride this horse at all.
Oxer, time to go back to basics. First, you have to make sure you're waiting for the horse; if you've had any tendency to jump ahead; this horse's jump will make you splat on the neck. Second, you have to make sure your body mechanics are exactly correct: if you've had a tendency to stand in your stirrup or lock your knee rather than allowing your knee and hip to fold; well, that plus this horse's jump equals splat on the neck, too.
So build grids and gymnastics and concentrate on waiting for the horse to push you out of the tack, and correct body mechanics. Grids without stirrups would be even better.
Lovely horse, good luck!