Grandchild being abused?

I think my 10 year old grandchild is being emotionally abused by his stepmom and her kids. Her kids tattle on and fight with my grandson all the time. They use him as an escape goat. He has told me that she has slapped him once. Her kids even told me that she is mean to him. He is like a male version of Cinderella. I don't know what to do. His mother abandon him when he was 3. I helped my son raise him until he met this woman. (She isn't really his stepmom, they live together) She even puts him down in front of me. I am afraid that if I tell my son, that he will get offended and keep my grandson away from me.