Hi there,
I would take your son to a child psychiatrist immediately. He is displaying behavior that is not at all normal, as you recognize. From what you describe it sounds like he may be a budding predator. If this is the case, it is a mental illness that can be addressed with medication, therapy and possibly placement outside the home, as heartbreaking as that is.
He needs help and he needs it fast.
This could escalate quickly. As he grows bigger and stronger, your son could attack you. As awful as it sounds, you do not want to become his victim. And you have two other boys to think about. They deserve to have a normal childhood. Not to mention, you have a right to a happy and normal marriage.
Keep us posted, others will also have good advice to share. I am sorry you are going through this with a child of any age, especially one so young. Please get him help immediately.
