How do I prove child abuse and get child support for my son?

Ever since my divorce 4 years ago my son’s lived with his father. He got custody because I was not mentally healthy when we divorced. I believe he is now physically abusing our son, he comes over to my house with bruises and won’t tell me how he got them. I know he’s scared of his father. His father was abusive to me during our relationship so I know he’s capable of it. If my son won’t tell me what’s happening over there, what do I need to do to prove that he’s being abused? If I can prove it and a judge finds the abuse happened, is that enough to have my son come and live with me and have his father pay me child support? I believe my son is going to need counseling to deal with all of this and I can’t afford that without child support.