Ah that's rough, poor lad. I wonder if there's an element of assumption that it's ok because it's "only" little girls doing this.
From a legal viewpoint, there's no order in place so you don't have to do anything, if you do not want to, it's for his dad to take it to court. The chances are, if he did so, none of this would be adequately addressed, let alone resolved, because court is a very blunt instrument, and very dependent on the individual judge and cafcass reporter you land up with.
What I fear you are stuck with is a father with zero idea that relationships are plants you have to nurture, hence his string of relationships, and the on/off crap with the current one. Unhappily, it takes a tremendous quantity of neglect and even cruelty to finally kill off a young child's love for its parent (survival stuff I suppose) so your poor boy has a world more pain to go through, whether you try to cut his father out now, or try to fight on to make him be the dad he should be.
No one can say with any confidence which course will be the least damaging for your son, because to do that you would need a crystal ball. It is however fair to say that you can't control his dad's behaviour, or make him change unless he wants to. Clearly he expects to passively receive his son's love without doing the active and protective work of loving him back, as if he is still a child himself.
I am guessing these women vanished from your son's life as soon as they were out of Dad's? He knows how disposable love objects are to his dad. Poor kid, no wonder he is so upset.