I would definitely go with circles. Transitioning downward as soon as they start to speed up can teach them that they can get out of working just by quickening their pace. I would say as soon as he increases his speed, pull him into a tight circle. As soon as he reaches the speed you want, release him into a straight line. This works with my hot headed Appaloosa, and it helped her understand clearly what I wanted from her. However, the cause of his increase in speed could be several things, from anxiety to a lack of balance. Try to figure out why exactly he wants to speed up.
I totally agree with Brenna! That is the way to do it! A few nice strides of canter and the moment acceleration occurs you ask for the downward transition. Eventually 3 nice strides of canter will become 5-6 then 10 +
Um my mare used to be like that and we had to lung her for 5/10 minutes before I rode.
Make sure you ask for the trot again before the canter gets out of hand. So only canter for a few strides, not laps. Also make sure to halfhalt almost every stride, and not to pull continuously but release and ask for him to sit up again, then release again.
I do heaps of transitions and leg yielding around the circle in the canter to get my pony to slow down 😊
Circles, lots of them. Also get them off the forehand