Do a lot of half halts and transitions. A good exercise is 10 steps of an accelerated canter to 10 steps of a slower canter. Do this by using half halts. You might also want to include going down to the trot and back up. It is also good to be counting the strides,like in the 10 step exercise, to help understand how big of a stride your horse has.
Do a lot of half halts.
Also a lot of full halts. Set up some cones or poles and stop the horse before, after or inbetween the poles and cones. Cut across the arena and stop at a random point. And do some backing up. And BEFORE you can feel him get ready to go go go half half and see if that slows him down, if it doesn't then put in a stop, if he pulls or tried to keep going then back him up and do a tight circle, then stop and make him stand for 5 seconds before he moves again. It can be a long process depending on the horse just take your time and try not to get aggravated!