If they specifically state that it was politically motivated, it can be a lawsuit.

Please stop saying this. It discourages people from trying to get justice and perpetuates the "slave to your employer" mentality.

https://www.nolo.com/legal-encyclopedia/employment-at-will-definition-30022.html

For example, if your employer is subject to federal and state laws prohibiting job discrimination (as all but the smallest employers are), you cannot be fired because of certain characteristics, such as your race, religion, or gender. (For more information on discrimination, see Your Rights Against Discrimination and Harassment.) Similarly, you cannot be fired because you have complained about illegal activity, about discrimination or harassment, or about health and safety violations in the workplace (see Nolo's article Assert Your Safety Rights Without Fear of Retaliation). And you cannot be fired for exercising a variety of legal rights, including the right to take family and medical leave, to take leave to serve in the military, or to take time off work to vote or serve on a jury.

At-will employment means they don't have to provide a just reason for firing you. It doesn't mean they can fire you for any reason. And if they do fire you for a reason prohibited above (such as exercising your legal right to protest), you have a claim.

At-Will employment means they can fire you for no reason, which is very different from "any reason". If they told him they fired him for what he said they fired him for, he may, depending on his state, have a case.