My gelding will sometimes buck while we're loping because he feels good (his saddle fits great). I usually just push him right throught it with my legs and keep him from dropping his head. If you haven't completely figured out the source of the problem don't go to kicking a day pulling at your horse because if he does it out of pain you'll only make the situation worse. If my guy doesn't ride through it right away I'll just keep loping until he quiets down. Also having his teeth looked at or having a chiropractor check him wouldn't hurt.
Be black and white with him- NO SHADES OF GREY! Meaning you don't let him get away with anything, if you say go right he better go right- make him do what you want him to, don'im waylk all over when he starts bucking, pull his head up and kick him in the shoulders HARD, then go back to what you were doing like nothing ever happened ( don't let him have the satisfaction of knowing you don't my phone is acting up very bad and won't let me delet characters, in one part I meant to put don't let him walk all over ya- and it came out totally different😂😭like him buck in g)!!
Have you recently changed to a new bit, girth, pad, or saddle? Horses want to work, they don't want to be bad or hurt you. Most of the time Horses react bad when they are in pain. 90% pain, 5% frustration, 5% Behaviors. Something could be pinching him, maybe you are working are something her doesn't understand, maybe trying to teach him something to quickly and you missed key steps
If your vet has ruled out a medical issue and it is not poor saddle fit, it is very likely a muscular problem. Many horses will act out when spasms become increasingly tight, pushing on nerves and the horse doesn't know how to handle the pain. If you can find a certified equine massage therapist in your area they can do wonders for your animal.
there are a few things that could be the reason.
Id check for if not only the saddle but all tack. Even a new girth can change the fit of a saddle.
Is this your first winter with him? It could be that he is simply more fresh than usual due to the cold.
Did his feed change recently? Horses are usually more hot when they are fed more feed.
Has anything recently changed in your riding? Is it possible that you may be landing harder than usual on his back?
Is it possible that he's confused by what you are asking? He may be getting frustrated by miscommunication and showing it through bucking.
Does he have a long back? Some long backed horses, show no pain for years and then suddenly start getting sore.