With a reaction like that to the whip you may want to think about desensitizing him to it first.. Did he only react when you tapped him with it? Would he do the same if you just rubbed him on the rump with it? Or if you tapped it on your boot to make the noise would he do the same thing?
My horse had a major reaction to a whip one day, and I'm pretty good at staying put in the saddle after my last horse, so when he managed to throw me as hard as he did I was rather suprised. He's fine with dressage whips, but heaven help you if you get on that horse with a jumping whip and even think about touching him with it.
Oh that reminds me... What sort of whip did he react to? Short whip or dressage whip or what?
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms