First of all, you need to give him moral support. When someone is the subject of a SJW storm, they ABSOLUTELY NEED MORAL SUPPORT. They will feel like the world is out to get them, that it's them against the endless hordes of the world. So get your ass out there and give him some support so he knows he's not alone in this and that people agree with him.
Second, he MUST NEVER apologize. An apology is an admission of guilt. He did nothing wrong, and apologizing would merely give the university the ammo they need to get rid of him. He must stick by what he wrote, and do so calmly and logically. This will create a contrast between his calm and the vein-popping incoherent autistic screeching of the SJWs.
Get him a copy of Vox Day's book, "SJWs Always Lie".