Circles and serpentines are amazing! Also for some horses I've had success with halting whenever they go too fast and ignore me. I would halt, wait a few seconds, back up a few steps, wait, go forward calmly.
Doing circles or bending the horse's head to the rail slows a bit.
Sitting deep might not work because most people drive with their hips when they sit deep during a fast canter. Try doing20 meter circles at every corner of the arena. Make sure he is on the bit and round and do not turn with your reins use your shoulders hips and lots of outside leg pressure
I would do lots of circles to keep your horse thinking about what you are asking him to do. While you are circling add half halts in and see if he is more responsive.