Stepson is so difficult

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My boyfriend's kid is an absolute nightmare. He's almost 8 years old and has been really difficult since he was 2. He has ADHD and Oppositional Defiant Disorder. Those two combined- make for an interesting experience. He refuses to read, write, do any school work, he doesnt want to do chores. We have tried so many reward systems and they just haven't been effective. When he is angry (which is often), he calls names, hits, and screams. He gets sent home from school a few times a month for bad behavior (hitting teachers, knocking desks over, spitting, running away, and threatening kids at school). He gets like this EVERY time he does not get his way! I met him when he was 4 and his dad has told me his behavior started when him and the mother started having issues at age 2. It's been blown off for so long as him just being a kid, but kids should not act like this ALL DAY every day. We have tried it all to try and get through to him! Charts, allowance, praising good behavior, ignoring bad behavior. I dont know what to do anymore. This weekend, he took a stick and scratched my car ALL the way around "just because". Im literally at my breaking point. My boyfriend and I just had a baby of our own 4 months ago and I'm so concerned about her turning out like her brother. Does anyone have any tips AT ALL for how to handle a child like him? He can be so sweet, but it's not very often and it is always followed by brattiness and craziness.

Thanks in advance!