Case worker said my nephew is already up for adoption and finalized but has not got adopted and I have no chance to get custody of him True?

Usually child case workers know the correct status of the children.  It's sad but could be true that the adoption has been finalized. If so, then you would have no chance.  You might check the court records or hire an attorney to help you?   You also may be relieved that your nephew got a good home.  You can ask the case worker if you can visit him?