I think the pose you have choosen looks too much like the ones that Mike posted above, it looks too much like the horse is bowing down, and not being ambushed. A horse won't bow like that unless trained to do so, it is a very unnatural movement/position for a horse. I have seen it done, and even well trained horses are very uncomfortable getting into this position.
If he is falling down, drop the right shoulder and tilt the front quarters down right so it looks like he is actually falling down rather then standing straight up and down kneeling like the pictures provided above. Also, turn the horses neck to the left and tilt the head to the left, to add some more dramatic elements to the pose showing the horses natural reaction to try to counterbalance the fall. Also, if a horse is falling down, he will be scrambling to get back on his feet, horses dislike falling down as much as we do! That is unless the horse is trained to fall on cue. So his back feet would more then likely not be so squared up and the left leg would probably be further forward in an atempt to regain balance.
As for the riders position, Mike is spot on. Especially if the guy didnt use stirrups he would be leaning back a lot to counter for the forward and downward to the right movement. Either he would be leaning back at around 45º degree angle or falling off leaning front and right off the horse.
But then again, maybe the horse is supposed to be posed kneeling! Then please disregard my remarks
All the best!!