Do a lot of trotting. A lot! When he achieves the right speed, you kind of do nothing. YOu let him coast along and if he wants to come back down, let him.
When he is good at maintaining a steady rythm at trot, then ask for a canter. If he rushes, don't grab his mouth, just make your body really heavy and think "Slow, slow, slow" breathing outward, and let him rush a bit and then find slow. You may need to circle him, but let him find slow. When he does, give him a long rein, a pet and praise and allow him to come to a halt.
Allow him to find slow.