That's a lot of horse to get aggressive!
Maybe a blood draw will show something. It sounds like you are doing all you can in the medical/chiro department. If there turns out to be no physical reason for his behavior change then I guess it's time to take the bull by the horns, so to speak, and instruct him as to who is running the show.
Please keep us informed and we wish the best for you and your horse.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger