Okay kiddo, let me explain what son assault demesne means.
Imagine you have a big toy castle called a demesne, and you are the owner of it. Now imagine someone comes over and starts playing with your toys inside your castle without your permission. That is called an assault.
If that person happens to be your son, it is called a son assault demesne. It's like he came into your castle and took your toys without asking, even though he is family.
Basically, son assault demesne is a fancy way of saying that someone, specifically a son, has trespassed on someone else's property without permission. It's not a nice thing to do, just like taking someone's toys without asking is not polite.