What can I do to protect my son and keep him safe?

My son lives with me. His mother has been using heroin and alcohol his whole life, he was bien addicted to methadone. He is now six. She's been in and out of treatment. Just recently went in to treatment for the third time in three months, left early the first two times with excuses as to why she left. She also has multiple DUIs, the most recent one two months ago on heroin. She has a blow and go for alcohol but it doesn't detect drugs. She has in the past drive high with him in the car. She also has a DV charge against her that happened when she left early from treatment. She has never kept to the verbal schedule we agreed upon for seeing him. Whether it was her asking me to pick him up early or her being late or just canceling all together. I worry for my son's safety while in her custody. We do not have a parenting plan as of yet but want to obtain full custody.