Hey,
we always taught horses who did not want to canter behind, first to canter next to another horse. You can start doing this in a school, get them used to ride to each other while you do your dressage. It is fun and can be tricky as the horses have to learn to go at the same speed once they know this fromm the school we did the same while hacking. And then you can slowly start slowing one horse a tiny bit down so that the other is in front with its head for example. We did this always slowly step by step and it worked fine with all horses
If he is young you can start doing it in walk and trot and then canter. If you practice walk and trot canter normally should not be a problem. But if you train its important to have an experienced horse which does not have a problem being in front, side by side or behind.
When I was teaching Khan we went out with a steady older horse. On a narrow track to start, we would have a little trot, with the other horse in front. If all was fine and under control, we had a few strides of canter, then back to trot. If anything untoward had happened, the horse in front would have 'blocked' the track. Once we had that going nicely, we increased the canter. Then we moved to side by side in a more open area, gradually moving in front, and also behind, so he was used to all variations!