Make sure he is nice and warmed up first get, do lots of transitions, circles and serpentine's, leg yielding, also if you know how to do turn on the quarters/forehand will get him listening to your leg aids, only when you feel his he is relaxed and supple trot a small 10 meter circle on the left rein, THEN give him the correct aids - outside leg behind the girth, inside leg on the girth and squeeze.....
Does he feel the same on both reins? Or is he stiffer on his left? You may want to practice transitions on the lunge?
Also if you have the money you could always get his back checked?