Mentally and learning disabled step-son

My step-son recently moved in with us, cause he kept running away from his mom. He is 15 years old but mentally it ranges from 4-8 yrs old. He has been neglected and undisciplined his whole life and now he isn't very functional. He can read or write. He can write the letters separately but it's more like a first grader. She gave him to us with no medication, SS number, birth certificate so we're having a lot of issues. His discipline issue are driving me crazy! She didn't teach or do anything with kid. It doesn't matter how many times we say it, he'll still act stupid, cause he's always been able to get outta things that way! He keeps doing things that make me angry, and I'm trying not to show it but it's become harder and harder everyday! I just don't know what to do!