Get a. good lawyer.
Unless he enjoy the protection of an employment agreement, either individually negotiated or collectively bargained, there is extremely low chance that would bear any fruit. With the exception of two states (one is CA and I think the other might be VA, but I am not certain), there is no protection for workers who have had adverse employment outcomes based on nothing but their political speech.
This is probably one of the least understood aspects of employment rights in the US. Yes, you can be fired for your political beliefs.
You cannot be fired for your religious beliefs, sexual or ‘gender’ identity (now, thanks to Gorsuch), your gender, your national origin or race or (in many cases) your disability, your pregnancy.