How lovely it would have been for your son to then be raised motherless.
Combating violence with violence is effective exactly 0% of the time. I find some of the comments in this thread quite disturbing.
There is an innate and primal part of every parent to defend their child. If a man slapped my son I would lose it, if he tried to sue me afterward so be it, but I would ensure he would think twice before laying his hands on someone elses child.
And combating violence with violence can be effective. In middle school there was a girl who picked on me all the time, she was brutal not only physically but she would say the worst things to me. I had it one day and beat her ass. She never messed with me again. There is a difference between violence and self defense, and if it wasn't effective there would be self defense classes etc.
You need to stand up for yourself, or in this case I would surely be the one to stand up for my son.