If he speeds up ride a circle until you like the pace of the canter and then go straight again. If they maintain it, slow down to a trot as a reward and then start again asking for a little more maintainance. Slowly build it up but as soon as you will go into a circle they will understand they are going too fast and it will take less and less.
Legs and half halts! Don't use too much rein or they'll get frustrated, just a little half halt as needed to keep them in check. :)
Also don't be afraid to let him open up his stride a bit when you first ask for the canter, and then use your seat and half half to get him to sit up
use your seat to help control and regulate your horse's speed. use your legs as well.
I'd say the same as Tessy, don't be afraid of keeping him with your legs even if he accelerates, and if he does, keep him with your back and your seat :)
Keep him with the legs! I know that's a hard thing, but keep your legs in contact with him and keep him with your back :)