Your letter raises several important and probing issues. Candidly, I am not surprised that the imposition of restrictions and punishment have not helped to motivate your son. Punishment seldom improves a kid's performance. Punishment does not eliminate misbehavior, it merely represses or suppresses it. Punishment seldom generalizes to other settings.
Punishment is only effective as long as the threat of punishment exists.
Learned Helplessness occurs when a child confronts repeated failure and frustration, After a while, the child comes to believe that failure is inevitable and unavoidable. Their response is, therefore, to simply stop trying. I am sure that your son's lack of progress is troubling for you. Your attempt to motivate him by punishing him appears to be ineffective. Maybe it's time to try another avenue.