Keep up with the lateral flexion, it really works. If your horse starts to walk off, just keep his neck bent and hold until he stops his feet and softens his face. He may walk in tiny circles for a while, but he will get it. You can do this on the ground or under saddle, with a halter or a bridle. After he gets the gist of the flexion at the standstill, you can send him forward at the walk and do some one rein stops, following through with a hindquarter disengagement. Lots of circles! Make sure he bends his entire body around your leg. Figure 8's and pole bending patterns at slow, controlled paces with correct flexion do wonders. My newest horse came to me knowing only that a rider is someone not to be injured. He was very stiff and had no idea how to stop or turn on cue. He is coming along wonderfully after about a month of flexing, circling, transition work, and groundwork. Hope that helps!
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown