Do you hold a whip in your inside hand??
It might help
Do what Kylie said and you can also try cantering a few feet to the inside of the track. That way when she's about to turn, you have space to push her outwards into your outside rein
During your warm up do a ton of transitions, as well as many different sized circles and movements. Then when it comes time to canter, pick it up as you normally do and halt the pony right before the area where she starts try to turn in. Then continue walking on and pick up the canter in the next corner.
Maybe set up a line of poles a meter in from the rail so that he maybe won't want to turn in as much.
Do he do this in both direction?