Help tw child abuse

So I found out that my son’s dad has abused him in the past. I don’t have any proof other than what my son says. Like haven’t seen the marks. I don’t have pictures. But he told me his dad hits him and stuff and it usually happens when his step mom isn’t home and apparently he does it to his 8 year old brother too. How do I go about dealing with this. He’s at my house now and I won’t allow him to see his dad. We have no custody written up. My son is 16. I’m in PA. Is there a number I call? Do I call my lawyer? I don’t have any money myself but I will do whatever the fuck I can. I’m so fucking heart broken.