am concerned for son, abusive wife, two small children how can I help him
If he is not willing to acknowledge this then you can only be as supportive as possible for him unless you witness, first hand, her abusing your grandkids, documentation will assist. Try to have another talk w/son if he heads in your direction for guidance, let him know this is affecting the kids, kids learn what they live in & this IS NOT good role modeling. If you involve CPS they will (possibly) remove kids from situation, this could be very traumatizing for them so please go slow & easy & document, record on cell phone, whatever you can do without placing anyone in further harms way until you must.
keeping the door open so he can come to you .............unless they want our help there i snot a lot we can do