My horse cannot do a balanced canter?

basically i have a welsh sec D and he is about 12years old and he has always struggled to canter to the point where he refuses because he cant do it properly i have been told this is because his back legs are too powerful and drive him forwards too quickly for his front to keep up with and then he is all over the place going faster and faster trying to get back in balance what can i do to help him ??? please :)