As schooling whip to just tickle (not smack or sting or whack or whip) his hind quarters. As soon as you don't think he gives an appropriate reaction to your leg. Don't be slow to use it. I used to see it in my mind as if it was a punishment rather than an aid. Now I see it differently and use it differently it is effective.
My mare has always been responsive but needed a little more pep. Using the schooling whip in this way has made a massive difference.
Transitions, transitions and more transitions! Praise when he moves off the leg well and a sharp kick if he doesn't or if you use a whip, a 'tickle' when he is extra lazy (as above has said). Quite boring but my mare soon gets the idea