Do as the Arabians do! The Arabs (people I mean!) would train their horses (Arabs too i guess) to stop if they fell off, because in the desert, if your horse runs away.....your toast!
They would attach a very fine but strong string (I would suggest a strong fishing nylon) to the horses head stall and practice falling off and hold onto the horse so that it has to stop and then when it stopped and approached the fallen rider (with some coaxing from the 'fallee') he would give the horse a treat.......therefore the horse would learn, 'rider falls off, I stay because there is this invisible string holding me, I approach rider, I get reward'......eventually this can be done without the use of the training string.
I would like to do this with my horse one day