Tell the father to get a voice activated tape recorder that the son can place in his room or in the house so when the abuse happens again, there would be a record of it.
I asked him something similar to this before. He said one has to be careful to record people without their consent.
Leaving a voice message is considered consent.
A witness listening in during an ongoing call can testify but it would be good for the caller to tell the abuser they are on the phone with someone who can hear. A child may not understand to do this.
I was shocked to hear a parent can actually go to jail for recording someone without consent. But now I am used to it.