As a grandparent of a newborn whose mother & baby tested positive for oxycodone what form do I need to get custody.

My son & his girlfriend tried to abort the baby themselves during pregnancy. The baby was born despite their intentions. My husband & I had no idea she was even pregnant. The baby was born positive for oxycodone, but he is perfect. The mother had the baby & left him at the hospital. When we(the grandparentss) found out she had a baby & left him we demanded full custody, the mother then said, nah I want him. She is still using drugs, my son, the mother & the baby live with us. The baby has our address. My son & the mother told us the baby had died after she left the hospital. We found out it was alive. We want full custody of this baby. As grandparents what do we do?