Here, lets put it this way; Would you keep a baby that was by your son?
I would not be excited at all about this "good" thing. It's messed up, if you had to sell the foal for whatever reason, no one (that has a good head on their shoulder) would buy it.
Lastly, I saw someone who was devastated by this. Now I see someone who is trying to argue that breeding a mare to her son is nothing dangerous as it's "done all of the time". 0_0 Make up your mind and geld that horse.