Have you checked his teeth? No kidding...had a mare a few years ago doing the same thing. Worse one direction then the other...simply refused to go forward at the canter when asked for a bit of contact. We did radiographs, accupuncture, ultrasounds, chiro work, blood tests.....etc etc etc.
Finally ended up doing a full body nuclear scintigraphy examination and it showed a tooth abscess that even trained vets specializing in teeth and three of the equine clinic vets could not detect with radiographs or examination. Who would have guessed it? The horse probably thought we were crazy working on her back and legs, joints etc when the problem was actually in her mouth. Tooth gone, canter issue solved 100%. Go figure..if only they could talk.