agree with what's said above ^ have a chiropractor come out and take a look. if this is something new, it's most likely pain related. but if he's always been that way, it could just be a training issue from previous owners (not to completely rule out pain )
If you rule out any physical problems, trot him up to a corner of the arena, and at the corner ask him to canter. Try only using one leg, and tip his nose to the inside of your circle.
Check for any discomfort or pain, check teeth, check the feet angles, and even your saddle fit. I'm not sure if your English or western but if your western this worked for me. This is explaining for a right circle. Tip there nose/head/neck to the outside of your circle and press with your inside foot. Imagine your pushing the rib cage to the left while the neck and hindquarters stay to the right.
It may be something physical.. were his teeth recently checked? Is your tack restricting movement.
Try steering your horse up the same way you do for the right but for the left, make sure you aren't leaning left. Focus a lot on bending and suppleness in your warm ups, and make sure you ask for sufficient half halts before asking for the canter.