Sounds like this principle has it in for your son. I hate thinking that someone in the education system could really be vindictive or just plain unreasonable but it happens. I would go straight to the board again and tell them what is going on. If they proved him wrong once, they can do it again. He has no right to talk that way to you and your family no matter what issues your son has in school. Once he "proves himself" he will have missed another couple weeks of another class. Unless he doesn't have the pre-reqs I see no reason how this guy can actually keep him out of class.

I would totally go over his head because you have tried dealing with him several times and it hasn't worked. Is there another school in the district that he can go to?