I'm not sure if you would've tried it before, but lots of transitions helped me a lot. The dull responses got slightly quicker each time, and it's really improved us as my horse resents the whip and spurs.
Practice at the halt no spurs. Bring your leg back and put it on with a little pressure. If he moves his hind end remove pressure and praise him. If he walks off halt him and try again. If he doesn't move at all take your leg off and put it back on. NO KICKING you must teach him to move away from PRESSURE kicking makes him even more dead legged. When he understands that when you put your leg on and there is pressure to only move his hind end sideways take it to a walk and push him in and out. Remember to praise him often. This works best with only positive reinforcement and no negative. While practicing this be sure to have steady contact and have his head down if not round. I hope this helps!