Anonymous wrote:So he demoted you, you decided to leave, and now is begging for you to stay at the current demoted level
No thank you
This. We have someone in our office that the boss and right hand made miserable for months. Granted, the employee didn't click, but they admitted her work product was good. Then, she gave notice and the boss begged her to stay. She stayed to get the project to the deadline, adding in an extra two weeks. I, on the other hand, would have given notice and not stayed a minute extra, if I had been treated that way.
OP, quite simply, don't let him bully you into staying. Accept you may have some skills to work on, but get the heck out of there.