MissAdventure
I presume that if the op said her son told her that..... anything at all, she would be advised to mind her own business, not go interfere etc.
Do "we" need the son to come online to confirm any of this in person?
One way or another nothing "we" say changes the reality of the situation--whatever it may truly be. The only people who have any say or power are the parents. The OP can be as concerned as she likes, her darling boy is the one who clearly isn't doing anything about the "situation".
Somehow I doubt a man whose mother overpraises him for simply doing what parents do every day is going to go to the courts and fight hard to get right of refusal.
If I truly thought my grandchildren were at risk, and my son was simply moaning to me about it rather than acting, my shame and disgust would be directed more at him than anyone else. I'd take my concerns to the authorities and let my son know what I thought of his lack of concern for my grandchildren's safety. I'd never want I child I raised to think complaining abut his ex is the extent of how much he'd fight for his children...if there was true concern. But hey, we're all different.