You definitely need to have the vet--preferably a chiropractor--check him out. He could just be green and not sure how to go about getting into the canter successfully while balancing a rider. You didn't say how long he's been under saddle. I bought a horse who, at the ripe age of 16, still didn't know how to get into a canter, and what he did looked like rearing to his former owner, so she quit trying. It took a while to teach him how to strike out with one front foot instead of two! Odd, but there ya' go. And I had a horse who we foaled and broke ourselves who, at about the age of 3, decided any request for a canter was an invitation to a rodeo. It took a really strong, confident rider (not me!) to get on him and ride him through that phase.
My advice: Vet first, then a professional trainer if the vet says there's nothing wrong with his back or the fit of your saddle.