I don't know anything about USA legislation regarding family law, I only have experience with it here in the UK, however, I would say though that you need to continue to report or at least record every single episode of abuse to the child that you hear of, dates, times and what happened etc and you absolutely have to get yourself a good solicitor.
If you or your daughter (frankly, it would be better if it were your daughter, at least that's how things work here) were to be awarded custody, you might also file for an injunction. Try to help your daughter be brave, if she goes for custody and he starts getting agro, she must call the police or report the threats every time and thus build up a portfolio of offences with which you can use as evidence.
The first thing to do is find yourself a solicitor (lawyer) who can fight on your behalf. Please do fight for your grandson, he sounds like he is in a terrible situation and he is helpless. Maybe you could encourage him to talk to a teacher at school who also might be able to file a report to the social services about the abuse he is privy to or receives.
I really do wish you all the best with this. thoughts are with you.
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