Thanks for your reply.

Truthfully? You have none. Washington is an at-will state; your son can be fired for any reason not prohibited by law, and this is not. They may legally fire him if they believed that he was sexually harassing someone, even if they are mistaken, and they have no legal obligation to provide details; in fact, depending on circumstances, it may be unwise for them to tell him anything.

It may or may not be unfair, but they have violated no laws.