It may just be my imagination, but he looks like he's swinging his back legs round instead of through and under him. Its kinda like what my dad does when he walks and he has arthritis in his hip so perhaps its arthritis/pain in his hip or stifle.
Also you say he can gallop, but not canter. If your lameness guy says he isn't lame (although gut feeling says he is :/ ) have you tried taking him somewhere where he can gallop in a straight line, and you gradually slow the pace until its a (fast) canter? Whats his history? If he was used in a cart, canter would have resulted in being told off, and being gaited means a double whammy against canter. Like others said, he may lack the ability to canter, or it has been 'trained' out of him