Lateral work has done wonders for my horse, exercises such as leg yield, haunches in, shoulder in, etc. But always have forward movement, think impulsion, not speed. Also try lots of transitions, working from walk to halt, walk-trot-halt, etc.
setting ground poles up to make 4 90 degree corners and then riding them at a walk, trot, and canter will really help your horse to get on his hind end. It's a little hard to explain, but i can send you a picture of it if you'd like!