There are two sides of the coin here - being defiant and being robotic.
Very often lesson horses learn to ignore cues, not because they don't like to do it, but because they're very confused. When a student has a death grip on their mouth but is kicking them to go, they have now learned that mouth contact does not mean stop. If a student yanks him around to the left, and he turns all the way, then yanks him around to the right. He's going to stop trying! Nothing he does is right! He has learned that nothing he does will relieve the pressure - which is his only chance at a reward.
There are 2 things I think you should do- first approach his 'broken' mentality. He sounds to me like he's just completely given up on trying. I think spending some good quality unmounted time, enjoying him, would do him wonders of good. Spend some time up on his back, not asking anything of him. Take him for an easy relaxing trail ride, don't ask much of him, just let him have fun. Find out what mounted thing he enjoys doing and do that.
Next you need to address his reluctance to obey. He has learned that nothing he does will relieve the pressure - so now you need to motivate him to do the right thing. Spend time working him lightly, but change things up constantly. Just light walk/trot, make circles everywhere, sometimes stop a circle half way and go the other way, make figure 8s and serpentines, turn and change and move all over the place. The entire time be SURE to relieve the pressure the moment he tries to do the right thing, even if it's not perfect. Make a number of transitions, walk/trot trot/walk halt/trot trot/halt walk/halt halt/walk and change it up constantly! Be sure the moment he does the right thing relieve the pressure. Personally I have NOTHING against giving treats so long as the rules are followed and he's never allowed to be pushy. Don't give him treats once you get off, give them to him when he does the right thing! If he does something that was tough for him before, or he wasn't doing right, just slip something into his mouth. Once he realizes something wonderful like that may happen he'll be trying hard the rest of the time looking for what he has to do to get that again! I just use hay stretcher pellets or tiny slices of carrots. I use clicker training though so I can click at exactly the right moment so he knows exactly what he did to get the treat. But you don't have to. I really do find that when horses realize there's something in it for them, they're much more eager for the ride :)
Got any pics of the pony? :)