Transitions, leg yeild, shoulders in, haunches in, and lots of circles! Your horse will fall apart, and that's OK, this will be very hard work for him. I speak from experience, my horse had and still has this problem, but he's getting better! Also something to do out of the saddle. Take your horse and walk him in tight circles around yourself, keep the pace slow so you allow him time to cross his inside hind over his outside as much as he can, and bring a whip along incase he gets lazy (tap his inside hind with it). Do this both directions before and after you ride and/or whenever you can. Also do lots of back ups, preferably up hills :) Hope he gets better!
"Riding is not Equitation... The moment you believe it is, is the moment you quit riding."