Confirmed child abuse allegations, and son made to go back to home of abuse.?

I am the non-custodial parent. I called the child abuse hotline after my son told me he was disciplined by his stepfather, that included being stripped of his clothes, put in an ice cold shower and made to talk while water was hitting him on the face. This allegation was confirmed by the mother and step father. Child protective service came to our house w/I 5 hours and did their interview w/us, first. I was forced to send my son back to the custodial parent. The investigator said the reason, "there were no visible bruises". My son (10 y.o.) told the investigator, the cop, and myself that he was afraid of his step father and did not feel safe. The custodial parent showed up at our house, and we were made to give my son to her and the step father, immediately. With much resistance on my part, he was handed back to the custodial parent. I called to talk to my son 24 hours later, I am not getting an answer but I am leaving messages! I was advised to keep a log of every single phone call that I make to them, so I am. My question is what can I do to immediately remove my son from harm, then get custody of him? What steps do I take 1st and What would be the best way to remove my son from the abusive situation so I can protect him?