Since he is (apparently) your nephew's legal guardian, the only way to stop him from that I am aware of is the get a restraining order and an injunction. These will not be easy to get. You need to hire an attorney to help you evaluate whether you have sufficient basis (and evidence) to get custody.
I agree with the other attorney, it will require court intervention but I'm not sure you have standing to file suit. If he's being abused CPS has power to remove him and place him elsewhere until court.
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