Join Date: Sep 2011
Location: Germany- but not German =D
Get him rechecked for pain. He could have twisted in the field or box.
Then, if he starts, either push in to a canter, and canter until YOU say when to stop.. When he tries to break to trot, push on!
Or, make it difficult to canter, carry on with trotting and push the trot forwards.
He may also be cold backed. Does he try it throughout? He may be trying to loosen his back off through canter. My advice would be give a good warm up, trot on a longer contact and push forwards. If he tries to mess around, half halt and keep pushing forward.
Horses will try to do this out of discomfort or pain normally.