Lots of pole work and raised cavaletti! Make sure you are keeping your leg on, you want to push him forward so to speak. Keep him energized over the poles and cavaletti. Also do lots of transitions like mentioned earlier. Keep him on his toes, keep your ride full of figure eights, circles, serpentines, spirals and direction changes. This will also help with balance and flexibility :) If he gets heavy, push him forward rather than pull him up.
Hope this helped,