My Partner Isn't Able To See His Daughter

So my (21F) boyfriend (21M) has a 3yo daughter with his ex and she had repeatedly ignored the custody agreement they have in place. She often tries to prevent his visitation with his daughter and ever since I've gotten pregnant she's gotten so much worse with insinuating that we should just replace their daughter with our baby and that he is nothing but a sperm donor when in reality he has done everything he can in order to see his daughter, even trying to be with his ex shortly after their daughter was born even though she was extremely abusive towards him. Now she has completely blocked him on everything and hasn't allowed him to see their daughter for over a month. What can we do because we are at a complete loss, we don't want to have to do anything that would traumatize his daughter but aren't sure how to get his ex to follow the custody agreement in place.