Sounds like half the lesson horses I've ridden (Being one of the the few in the barn that didn't lease or own, I got shuffled around a lot and put on all the new guys.) Really, while his attitude isn't unusual, he does seem to be going to unusual lengths to stay lazy - almost like he's working at it hard enough he's defeating his own goal of being lazy!
Thank goodness he ended up with a rider with a seat and the determination that you have to keep him honest. I think you are right, if you know it's not a physical or physiological issue, to keep after him and thereby maintain consistency with your expectations of him.

A horse may be coaxed to water, but a pencil must be lead. - Stan Laurel