I grew up around TWHs, and the rack was a natural to them as trotting is to other breeds - the babies would go racking across the pasture like it was no one's business. It might be more that he hasn't been consistently taught to rack instead of trot while being ridden, but my bet is he can rack.

Might try asking him to increase his speed while trotting, but not let him break into a canter and see if he'll pick up the rack. Not sure what else to tell you. I rode them all the time as a kid but never did any of the training!
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PH Cody
HorseHobbyist Site Coordinator