Ponies are very clever and I've seen a lot more "fighty" than "flighty" granted that they haven't been abused.
When feet get stuck, I go back to lunging. Just because you carry a whip doesn't mean that you have to whip the stuff out of a horse, its just an aid to your body. Use only as much pressure as necessary (funny thing there, I was almost going to say force! Lol, had to be there). Don't be surprised if he jumps immediately into flight and charges around the circle. It is very normal for horses to jump from one extreme to the other for a while, but it will be less severe each time.
Once he is pretty consistent with go and stop on the line, I would move him to long lines. This will help with understanding direction and rein aides, as well as cues from behind him.
If you overprepare a horse for riding before you climb on their back, then all they will have to learn is how to carry the weight of the rider. It is only normal that if there has been some confusion or rushing in the previous training, then it is going to show as soon as someone crawls on his back. Not because he is trying to be bad, but just because its mentally and physically confusing for him.
Disengaging the hindquarters without proper flexions is not healthy for a young horse, or any horse for that matter if they don't have the training through the hind end. Also, I don't want to fix problems with making the body so it can't do it, but instead figure out why they won't do it and fix the problem there.
People get very caught up in the idea of "teach the horse a lesson, show him he isn't the boss". The reason that circles work is because its a form of travel to help the horse achieve natural balance that has been deprived of him due to his time being domesticated. If traveling is comfortable, then I have yet to meet a horse that wasn't willing to do it. Horses have safety as their number 1 priority, comfort as number 2.
Ponies typically get thrown under the bus due to their intelligence and small size. They always get the "pony syndrome" label when really their main focus is self preservation. Not only that, but they are usually destined to carry around small children, who may be the sweetest things, but rarely have the horses comfort in mind. People rarely spend the time training a pony that they put into their own riding horses, and then call the pony stubborn and ornery when the pony doesn't want to cooperate.
This isn't a battle to win or a situation of "do this so he can't do that". Its a matter of training. When training a horse, weight bearing is actually the last thing that they do, since this immediately compromises their flight mechanism and requires a lot of trust and understanding from the horse. If you give him the tools he needs to sustain his physical well being while under the weight of a rider, then even the most "stubborn" pony will follow your leadership willingly.