But he IS clinical. All the behavioral and physical symptoms you’ve mentioned are clinical symptoms of KS.
Your chiro has noted SI and back soreness - why wouldn’t you immediately fixate on the KS as a possible cause?
OP, I made it through your whole post wondering when that was going to come up. I wondered a few other things about the management mentioned. Try turning him out in a herd. He is a baby, he needs social time with copacetic herd mates. Involve a professional to help you work through the ground manners problem. Continue treating for ulcers and consider scoping, but remember ulcers don’t resolve well if there is underlying pain.
How about his feet? My experience with OTTBs is they need a multi-pronged approach therapy-wise the first few months after retiring from the track. The track uses up their body hard. Chiro, massage, even some short-term pain relievers, aggressively addressing their hooves, unlimited turnout, and lots of good forage to get them back to feeling their best. It can take years to undo physical damage from racing.