Soft seat, lots of leg to encourage the horse to move onto the bit and accept your contact, and hald halts on your outside rein. Reward with give when your horse slows, don't hang on his mouth as he'll likely just get faster to try and run away from the pressure. Good luck! Hope I could help <3
If your horse rushes in the canter, do lots of circles and turns to help him improve his balance and slow him down, since unbalanced horses rush in order to steady themselves. Circling engages their hind end and makes them put their back legs underneath them to steady themselves.
If he won't slow down in the canter, despite you giving him clear instruction to, maybe a change of bit will help (try and avoid using a stronger bit - this will make the situation worse). Horses that don't like the bit tend to rush away from your hand in an attempt to escape from the discomfort in his mouth. It also helps to have a different cue to slow, such as 'woah' or shifting your weight back in your saddle, rather than relying on the bit to slow your horse.
Finally, lots of transitions from trot to canter help a bunch as this gets him listening to you more and he will eventually slow when you ask.