He looks to me, as if he does not want to bring his back end along with his front end. It looks like he's doing all his work from the front and the back is tagging along, against its will
My Arab has been like that since I rescued him 19+ years ago. He had an injured vertebra. He never could be an adult rider's horse, so ended up being a lesson horse for children.
So perhaps something in the skeletal structure is being missed by everyone and that segways right into "kissing spine". Of which I know nothing about but I know it's easily missed and could that be a possibility?
If he's young enough, perhaps it's not to the point where riding him provokes bad behavior. Kissing Spine
He either has an inherent anomaly that isn't being discovered, or he's been injured somewhere in his time that nobody was aware of and this is the result.
Being a Paint, I know there are certain genetic diseases they can inherit. I wonder if finding and joining a Paint Horse forum might get you anything??
I dunno, I am stabbing in the dark, since you have already done everything you can think of to figure this out.
And yes, he is absolutely gorgeous