Is your horse registered? What's his registered name(I'll look him up)?
A natural gaited Saddlebred is almost never shown in five gaited classes. (I know of one showing right now who came from the very competitive speed racking circuit who is now in 5G classes). In any Saddlebred the trot is the most important gait. Only a few are "worthy" of being five-gaited. A natural gaited horse will have the tendency to mix his gears, and might get to the point where you lose the trot. Essentially, it is easier to teach a trotting horse to rack than a gaited horse to trot(horse in this instance means ASB). They all have the natural ability to learn to rack, though some are better suited for it.
I can think of a few things that might be partially to blame for this canter issue. Pain. Unfitness(being lazy). Confusion.
If you are not asking how the horse knows, then he might just try anything until he gets it right. I know a gelding who gaits whenever he gets confused, or worried. "Ahh!! I don't know what you want so... slow gait!" Horses are lazy creatures. They will always try to go back to what is easiest for them. My five gaited mare, would try to break into a canter when she got tired or lazy at the rack. "This is WORK. I'm going to canter." And of course pain. It might cause discomfort or pain somewhere to balance a rider and canter. I would check back pain, and I would also check to see if he had ever been foundered or has any pain in his feet, as this could also discourage the canter. It would discourage the trot too. It is just something to add to your check list. You could try giving him some pain meds and see if that helps. If it gets better, you may have found your root.
As for feet, if you want to discourage the rack, look into your angles and length of foot, front vs. back.
Were either of your horses(the OP and the other one who posted about canter issues) started as saddleseat horses? Knowing that might help.
Other than that, I'd say post your question on Trot.org. There are many people there who can help you with your issues including many trainers of all seats, who have trained and transitioned Saddlebreds for and from all different areas. They can tell you just about anything you need to know.
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