Dear readers: My wife doesn't like our son

From a reader,

I have three children with my wife. Two of our children are girls and the last who is just three years old is a boy but I have noticed that my wife treats him differently from his sisters. She's not patient with him and smacks him whenever he does something wrong instead of correcting him in a loving manner like a mother should. She also talks to him harshly. If I hadn't seen her give birth to him, I would have thought he was adopted. I have asked her many times why she treats him differently but she says I am just exaggerating. I knew this was getting out of hand when I contemplated getting a camera to see how she treats him when I am not around. That is his mother not his nanny so I shouldn't be scared to leave him alone with her but the sad truth is that I am. What should I do?