Help for my stepson-mental abuse?

My stepson is 3.5years old. he lives with his mother who is severly mentally ill(social services are involved). they have issues with her care and treatment of him and we want to know why he cant come to us as we have no issues in our home. she currently has called the police on us for no reason and the child is now inflicting injuries on his pets.she regularly tells my husbands son that he is not his dad anymore and he is always on the phone to his great grandmother totally distraught..we have told her we are going to arrange therapy for him but she is refusing. what can we do???