My son is being neglected what should I do?

My sons mother has full legal and physical custody. I just found out that he is not living with his mother and has been living with his grandparents since he was 18 months old. He is now 4. I talked to the grandparents and she gives them nothing for food clothing or housing for him. He has never been to a dentist and has a cavity, and has needed gles for 2 years and nothing gets done by her. He and I are being fleeced by her, she is using him as a meal ticket. Any suggestions what I should do? I just found out she has no running water or electricity at her home either. My son doesn't live there in reality, but as far as the govt is concerned he does... what should I do? I don't necessarily wanna take him from his grandparents since they are all he knows, but I do want to make sure that I have control over his future so I can make sure he is taken care of. HELP!!!