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.
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