"Whoa" and then backing will definitely get the horse on his hind end. Once you have "whoa" down, start some rollbacks. I always loved them and they will definitely build up the hind end and give your horse something interesting to try.
Ride about 4 feet away from the fence. Ask for "whoa". Once the horse is completely stopped, turn him the other way, but turn him into the fence. Since you are turning him into the fence, he doesnt have a choice but to sit back on his hind end to turn instead of walking a circle. As soon as he has made it past the halfway point of the turn, then ask him to trot off. You want him to propel out of the turn. Then you can repeat the steps but turning back the other direction.
You can eventually do this all in one motion pretty much. Ask the horse to "whoa" and this will be almost instant that he will stop and you can in one fluid motion have him do a rollback and canter off.