Slapping children/ DHS?

I live in Iowa. I called DHS in woodbury county and reported that my son who is 8 was slapped by his mother so hard that he has hit the wall or falls to the floor. the DHS worker almost laughed after I told her what he told me. later she wouldn't even do a report, she said if there was no bruise there is no abuse. the mother beat my son on a reg basis, when he was 2 and under. he came to my house black and blue and with unexplained burns. when I took him to the hospital and they contacted DHS, the worker determined that it was unfounded (they won't do anything) and told me that if i continue to report these incidents that they would take my son away from me. later i reported more abuse and got another unfounded report. they told me that "there IS abuse happening in her care but we do not know for sure that it is her doing it." if a child is in her care and they are abused and she doesn't do anything about it, isn't that neglect which is also abuse? It hurts me to see my son hurt but I can't take him and run or I would be thrown in prison and he would go back to her, and DHS won't pull their head out of their a** and help him. What can i do to protect my son? and how do i get action out of DHS?