I have found it has a lot to do with the rider. Count one two one two to the beat of a metronome. Slow yourself and your horse should come down with you. Also sit deep in your seat. When we do our transition from our large fast to small slow circles in reining we just sit back a bit slow our bodies down and the horse slows down all without pulling on our reins, long drooped reins is our key
Half halts into the canter and small but not too small circles and try not use a lot of reins maybe more body like a pelvic tilt when you sit more in the saddle and not lean forward because if you lean forward your horse tends to go faster I hope this helps
He may be trying to rush the canter so much because he has a weak hind end, or simply because he doesn't want to use it. Try to do exercises to build up his hind end. Doing cavalletti work and transitions can help to slow him down a bit and will build his hind end.
Working on shortening and lengthining the stride and doing lots of transitions will help!
Probably loosing balance and falling on the forehand. Thus speeding up to try and get his or her balance back. If you are not a very experienced rider, I would only canter a few strides at a time until your horse gains balance and muscle. If you are more experienced, then you can use your seat and half halts to rock the horse back on his hind end.
I'd work in a circle, half halt every other step, and not canter for too long so that you are able to bring them back to the trot before they run away. Slowly you can start to let them canter for longer, as their canter becomes more balanced.
Work the canter in a circle, always the circle helps to slow down because is a more demanding exercise, when your horse is calmer, you go straight, and if your horse starts to speed up, make another circle
My horse does that but if you pick your hands up higher it will help them get more on their haunches so it will be easier to stop
Lots of half halts and a low seat worked for me when I was having the same issue on a green mare, or you can try pulling the horse into a small circle which will force him to slow down. :)