I have been married and living with my husband for 3 years. I now have a 6 year old stepson that husband has?

had full custody of for over 4 years. The mother is no good, has been in and out of jail and on numerous drugs and we have no contact with her at all. I guarantee she doesnt have a job and I am wondering if my husband could ask for child support and how that works since she would probably not be able to pay? Taking on a child is a big responsibility and I don't think its fair to my husband or I that she get away scott free.