Leg and then more leg! Once you get him cantering no matter how ugly you may feel you must make him keep going. Kick, use a crop whatever works. Give yourself a much larger space in order to get the canter, a dressage arena size isn't enough if it takes you half the length of the arena to actually get the canter and then you have to turn sharply. But make sure you are not accidentally telling your horse to go back into a trot with your reins. Be conscious of the fact then when he canters you could be using the reins to balance through the transition which is then telling him to stop. Make sure you are very clear and precise with your aids and if he doesn't respond to that; is when you ask harder. Don't be afraid to use a crop, your horse has to learn to keep going.
Use your seat to push him forward.
Lots of leg, squeeze with a beat, crest a beat with the canter and squeeze with it, if that's not working you can even do tiny little kicks, not big kicks but little ones. If that still doesn't work give him one or two huge kicks. And if that still doesn't work then I would pull out a crop and use that
Main advice is to keep your leg on and stay in your seat