I believe that will be CPS' call. Your son should be going to court to seek custody, physical custody for sure, and perhaps sole legal custody based on her neglect. He should let CPS know he is doing that. He should take initiative so as to avoid CPS putting the child in the foster care system away from them both.
To questioners from West Virginia & New York: Although I am licensed to practice in your state, I practice on a day-to-day basis in Massachusetts. I answer questions in your state in areas of the law in which I practice, and in which I feel comfortable trying to offer you assistance based on my knowledge of specific statutes in your state and/or general principles applicable in all states. It is always best, however, to work with attorneys and court personnel in your own area to deal with specific problems and factual situations.
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