I am fuming! I would say if you can afford it get an attorney, and fight these lowlife socialists. Please tell your son that he did NOTHING wrong. This is beyond the pale.
We had something similar happen to us. We hired a lawyer. We are now thousands of dollars poorer.
I'm sorry to say this, because I was once idealistic about fighting city hall (so to speak). BUT DO NOT HIRE A LAWYER. A lawyer in a case like this will only fleece you, piling on the "billable hours" while getting you nowhere. With public schools, the deck is so stacked against you, you will not get anywhere.
If you can get a conservative/libertarian group to help you -- such as HSLDA, as someone suggested, more power to you. You could try going to the websites of the Rutherford Institute and the Center for Individual Rights -- they sometimes help pro bono with cases like this; but their funds are limited and they will usually help out only if they think your case is of major constitutional significance.
Oh, and btw: We homeschool now.