I think "fighting words" is still a real thing in this sense: If I go into a bar, and say the wrong thing about someone's mother, the law is not going to be surprised that they throw a punch at me.
But in our current culture, it has to be narrow. Otherwise, Antifa is going to claim "fighting words" justifies them brawling in the streets. And, legally, it does not justify it at all.
You're incredibly mistaken, and if you act on this misinformation, you're going to find yourself in prison for assault and battery. The "fighting words" doctrine has been repeatedly narrowed over time by subsequent decisions (and may not even exist anymore), and never excused any violence in the first place. It was an exception to free speech, not the provisions in the criminal code that prohibit violence.
He'll be imprisoned for insulting someone's mother?