Child custody requires that the court review several statutory factors. The situation your describing certainly would be one that I think, if these facts were proven by a preponderance of the evidence (little more than half), would necessitate a change in custody. See the article linked below oldie further discussion. You should seek local counsel to assist you in a complete review if your case.
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I am not sure from your question if a custody order exists. If so, The child's mother will need to file a Petition to modify the current custody order. If she has sufficient evidence to prove that the child has been neglected by his father, it will strengthen her case. Because of the child's age, the Judge will likely speak to him. He may be able to provide important evidence of neglect as well. The mother will need to hire an attorney to file the Petition or Complaint if an order has not yet been entered.
My response is based solely on the limited information contained in the question. It is not meant to substitute your attorney's advice.
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