How can i teach my off the track thoroughbred to canter!?

You need to go back to square on with him, beginning with riding in an arena at the walk. He needs to be retrained (with a lot of patience) to yield to light bit pressure and to respond to your seat and leg. The best methods to do this are those of classical dressage principles. Once he responds in to your seat and leg and light hand in the walk, you proceed to the trot and then eventually to the canter. This is a slow, methodical process, with the horse letting you know when he's ready to progress to the next step. What you DO NOT want to do is pull on his mouth with both hands. You will start with one rein only to begin to teach him to yield to the pressure rather than to lean into it. This is just the beginning and it would take far too long to go into detail about the whole process here. Get a copy of one of the classical dressage texts (one of my favourites is Riding Logic - Littauer) and study it. Be patient with your horse. Don't be in such a hurry to get him cantering. The more time you take now, the less time it will take later on to get a good, calm, balanced, rhythmic canter.