What would you do in this situation

My son's close friend's dad is really abusive. He abused him in front of people. He hits him, calls him a fucking retard and is just a disgusting man. He comes over almost every weekend just to get away from his dad and he always has a new bruise, or black eye. Thing is, even with all the violent abuse he is the most gentle teenage soul I have ever met! He is so kind. My son actually became friends with him because he was sitting all alone and he was bullied a lot in middle school and my son witnessed people call him weird and retarded because he has Asperger's. So my son befriended him and they've been friends for 3 years. I really want to help him because his dad is always beating on him but when I talked to him about it, he had a full on panic attack begging me not to call DHS because he can't go back to the foster system. Just talking about the foster system put him into a crying panic attack. Idk what happened to him in the foster system but for him to be okay with being abused if it means he does have to go back it must have been really bad.