What can I do to get my son back into my custody from his now ex-step mother's care?

Back in late February, early March; my ex husband (by common law), his then fiance, and I went to court about temporary custody of my son. It was agreed upon verbally in front of their lawyer that she would have temporary custody till I was able to get my own place (I was living with my dad at the time) while my ex went to rehab. What actually hit the court papers was that him and her have joint custody and I can only see my son at their discretion. Since then, she has moved (not notified the courts), she has left my ex (so she is no longer his fiance), and refuses to allow me or my son's father see our son very often (and when I do see him, he BEGS me not to make him go back). She has mentioned to me more then once that she was going to allow her older son beat up my son for different reasons. I can not afford an attorney, I have already called the cops to do a welfare check, and my ex has already tried calling CPS. What can I do, legally, to get my son back into my custody?