If the saddle fits then the next step to rule out pain (and correct it) would be to have him adjusted by a chiropractor.
That being said, he should still be expected to react to pain or discomfort in a respectable manner. Kicking out is not being respectful and he should get disciplined for it. If he moves away to avoid something causing him pain, that would be a more appropriate and respectful response. Keep this in mind when you work on correcting his behavior while saddling.
Now, to correct his kicking out while saddling, I would take him into the arena without having saddled him. Keep hold of the lead rope yourself by laying it over your arm at your elbow. Don't tie him during this process. If he wants to move away from the saddle, he should be allowed to at this point. Your hands are free to pick up the saddle pad and saddle as long as you have the rope over your arm, plus you will quickly be able to reach out to grab the rope if needed. First take the saddle pad and toss it on his back like you would normally do. Does he have problems with this step? If so, take the pad off, maybe rub it on him a little bit, maybe go to the off side and try, then toss it on his back again. Any time he has problems - walks off, kicks out, bites at you - you will need to start over. The kicks and biting should always have a reprimand. Walking off should not be disciplined (at this point). When he is good for the saddle pad, then you can re-introduce the saddle to him. Bring the saddle close to his side, like you are about to put it on his back, but don't put it up there yet. Is he ok with this? Or does he see that saddle coming and then start to be disrespectful? Biting and kicking need to be strictly disciplined. Any sign from him that he is not accepting the saddle means that you have to start over (including the saddle pad). If he walks away, start over. If he kicks out, discipline him and start over. Heck, at this point, even if he does stand to allow you to put the saddle on his back, I would reward him with a little scratch at the withers, then take the saddle off and start over. Notice that we just have the saddle on his back - we did not even think about tightening the cinch yet. When he stand for the pad, and then the saddle, only then should you think about reaching underneath for that cinch. Because you are going to follow the same process - any time he shows you that he is not accepting & respectful of the pad, the saddle, tightening the cinch, etc., you need to take everything off and start over.
It is a desensitization process and it will take time. If this is the only thing that you do with him for an hour or two everyday for the next two weeks, then that's what it takes.
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