Well I think that the fact that your pony has only been ridden English might be your problem.
What I would do is that you get the saddle put it on the horse. Take it off. Put it on again. Let the horse see the saddle. Take it off, and put it on again. Let him get the idea "This is a saddle, it isn't going to hurt me."
Start to lunge him (without a bridle) do a walk, trot, canter. Do a lot of turning let him get used to the feeling of a western saddle. Western saddles and English saddles are completely different from one another! Western saddle can be heavier than an English saddle.
Just lunge him with it on, he might buck and do what you said he would be doing. Do a lot of turns let him use his 'thinking' side of his brain rather than his 'reaction' side.
His reaction side of his brain would tell him this, "There is something on my back that's never been on there before! Get it off!" And hes going to buck and try to get it off.
Lunging him and using turns and desensitizing the saddle with him will get him to use his thinking side like this, "Hey, its just a saddle is isn't going to hurt me or anything."
Lunge him for awhile, once he finally gives into the saddle and he realizes it isn't going to hurt him (you can tell this by when he licks his lips and he does whatever you say without hesitation, and he doesn't buck, ect.) stop lunging him, get on, and ride.
If he starts to buck again lunge him, and if he finally gives in and you get on him and he bucks again. I would just start out at a walk. When he doesn't buck or anything say 'good boy' make it positive.
Then start to trot if he doesn't buck say 'good boy!' and pat him if he bucks make him do circles. He gave into the saddle, now he is probably just testing you. Once again get him to trot, if he doesn't buck say 'good boy!' and pat him.
Then canter do the same thing as what I said above.
Lunge him everyday with the western saddle. Praise him if he doesn't buck.
Good luck!
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EDIT:
If he bucks with his ears not back here is an explanation....
Horses have to sides to their brains. Reaction (which is what they do in the wild because they are prey animals) and thinking.
They use their reaction and thinking side like this: A horse is eating grass in a field, there is a loud BOOM! The horse automatically runs because he thinks there is a predator there ready to eat/attack him. Then after that he uses his thinking side "I've got to eat to survive" and he goes back to eating.
If a horse uses his thinking side mostly and he is out in the field and hears that great big BOOM! If he uses his head he would be like this "There isn't any predator, its just a noise that is coming from the neighbors" and he will continue eating.
Do you see where I'm going with this?
Anyways your horse is using his reaction side when he bucks.
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Horses pin their ears to pressure.
For example a mare is eating grass and she doesn't want anyone to come near her. If a horse approaches her she would pin her ears first, saying "i don't appreciate your presents". Then if the horse ignores her she would snarl her lips and pin her ears saying "Did you hear me? Go away" if the horse doesn't leave she would lunge at him to get him or her away. Its pressure.
Sometimes horses pin their ears when they are bothered or hurting.
Do you see now? Your horse is using its reaction side of its brain to get the saddle off. He isn't familiar with it, he isn't hurting, so he isn't pinning his ears. Its more of a "Why is this thing on me?!" And using his reaction side to get it off rather his thinking side saying "Oh, its a saddle, it isn't something that could kill me."
Horses are naturally curious. So hes going to be wondering whats on his back. Hes going to want to get it off of him. He just needs to use his thinking side.
Hope that helps!