Since you have ruled out soundness issues, try simply shifting your left pelvis or hip toward 2 or 3:00 (imagining your saddle is a clock face). This will help open up his right shoulder and help you push the left rear leg into the canter lead. Other suggestions above should also help. Your horse is probably feeling a bit unbalanced and not used to that lead and it will take some time to develop muscle memory to help make him comfortable on that lead (like you using your less dominate hand to write or eat with). Be patient and try to not get frustrated. Your horse is probably frustrated to when he can’t do or understand what you ask. When he does get it, be sure to praise him lots and try not to overdo each session. He’ll come around. Best of luck.