Your guy is cute as a button! What a sweet face!
I have a TB cross (75% TB, 25% Holsteiner) and he has mild KS. His first indications were similar “bumps” on his spine and explosive bucking behavior. It’s good to remember that horses run from pain, and he would bolt or break into the canter at seemingly random times. He also had a tendency to work “up” instead of get calmer and looser as the ride went on. I’ve since heard that is pretty classic KS behavior. As someone described above, it becomes a cycle of inversion and pain. My horse was diagnosed with field x-rays - he showed some remodeling on T11-12 and T16-17.
The good news is, it can be managed. I think people are afraid of it as diagnosis (I was too!) but the truth is that correct exercise helps these horses tremendously. We don’t jump anymore, but my guy is 19 this year. We mostly focus on dressage. He gets weekly bodywork, and on days when he is “ouchie,” we just take it easy and hack out, or do lateral work at the walk.
If he were mine, I would keep an eye on that hock too! The whole hind end, including the palmar angles in the feet as someone else mentioned above, can exacerbate any back issues. I get fairly routine X-rays of stifles, hocks, and feet on my guy just to keep an eye on everything.
He’s lucky to have you searching for answers. Good luck!!