Thank you for your question, Amanda. The first step is to have a vet give your horse a good physical exam. It sounds like there may be something making him uncomfortable when he canters; this gait puts more stress on certain joints.
Assuming the exam finds him healthy (I will avoid the obvious pun of “…as a horse!”), I would suggest retraining him to canter. It is important for you to be calm and relaxed. I suspect that at this point, you both have a fair amount of anxiety when the canter cue arises.
In an arena with two long sides, trot the short side, and through the turn place your outside leg behind the girth, lean a bit forward and verbally cue him into a canter (I use “Step up”) softly, calmly and slowly. When he transitions into a canter, you will reward him for a smooth transition by slowly and softly sitting back and giving a verbal cue, such as “Whoa.” When he transitions back to a walk, keep a loose rein and reward him with a scratch on the crest of his neck. Repeat these steps, slowly increasing the strides he takes in a canter. If he crow-hops, simply say “No,” transition back to a smooth trot and try again.
By slowly and patiently increasing the number of strides, and calmly redirecting him when he defaults to his old behavior, you will likely be very successful in modifying his behavior.