So the vet was in today to float his teeth and give him his Strangles vaccine since we are travelling to a trainer soon and she requires it. I asked him to give him a good go-over and the vet thinks he's in perfect health. So I really don't think he's resisting lunging because of physical issues which magically do not manifest themselves when a rider is on him. I think he just can't get excited about it the way he does about jumping or riding or heck, even groundwork is something he enjoys more than walking around in a circle.
He is far from green. He does not need to blow off steam before being ridden. We've already determined that he is sound. He has no bad habits or issues we need to resolve. He WILL lunge, but isn't throwing himself into it with the same level of enthusiasm. And yes, I am using a verbal cue for the canter (kissing noise). I am upping the pressure by making myself bigger, more energetic, pushing him with my body, agitating the whip, but still no canter. I think I would have to actually run over and smack him on the rump. Yet, when he's got a rider on him, he happily canters on the first ask.
Maybe I'm doing it all wrong. But overall, I'd rather ride him than lunge him and he'd rather be ridden so... I guess we're going to ride unless it becomes necessary to lunge again for some reason. Looking forward to a trail ride on Friday.
I really appreciate all your thoughts.