There’s always the chance they won’t take his bad mouthing seriously, especially if they know how things have gone between you. I understand wanting to control this narrative and deflect his negative energy away from your friendships, because it feels like he’s trying to steal even more from you. Bottom line is, trying to control what he does/says is a waste of your precious energy. You can’t gag him (sadly) but your true friends will see his bad mouthing for what it is - a big bunch of sour grapes!