I don't really know what more you can do seeing as how the police are already involved. I wouldn't push Josh to talk to you, but make it known that you are there for him if and when he wants to talk.
I'd tell your parents. First of all you were there and witnessed his parents abuse. You may at some point be asked to fill out a witness report, if so it'll be better that your parents know the situation ahead of time. Also if Josh calls late at night because he needs to talk your parents may be a bit more understanding if they know the situation.

When I was in high school I had a friend who was abused by her parents and she was taken out of their care and then adopted by another family. The dad in that family then abused her. She came to school with the most unbelievable bruises and cuts. She would cover it the best she could with make-up and long clothes. I knew about it as did a few other of our friends. She didn't want to be taken out of that home the way she was taken away from her birth parents so we said nothing. Then one time she came to school so badly beat that I had to do something. I went to the school counselor and told him the situation without giving him my friend's name. He told me that if it was as bad as I was telling him that he needed to know her name, I told him. Child services were called and my friend's father was arrested that day and she was pulled out of that house and placed with another friend's family. They were good people and she fit right into their family. She never did go back to live with her adopted family, but she did see them often and she was happy and safe with her "new" family.