15 yr old son won't go to school

I don't know what to do. He's always gotten poor grades. I've helped him with homework for years but that doesn't help him take tests. When he applies himself he does good but he doesn't seem to care anymore. He failed 4 classes last 9 weeks. He thinks he will still pass. He says they don't hold kids back anymore.He is on probation for harassment of another student. (Phone calls and texts) his po at the school says she can't do anything about school just getting him his community service hours and talking to him.His older sister did the same thing. Now she has no job at 25 yes old with a 5 yr old son. She was a heroin addict and stole from me and him and after she went to jail she went to rehab and now she is on subs.He says he is not doing drugs but I don't trust my judgement anymore. I can't make him go he is bigger and stronger than me. He has 2 other older Sisters none of them went to college and they are both on their own working at low paying jobs.Im failing him.i have no help from his father who abused us both. He doesn't go to visitation with his father and that's fine with both of them. Father is an alcoholic and drug addict.I have requested help from a crisis organization and they are helping me get a Medicaid cars for his therapy which he says he won't do but they say the therapy will be for me to figure out how to handle him.He's been diagnosed with ADHD mixed presentation, oppositional defiance and depression unspecified. He threatens to kill himself a lot.He has a gym membership and plays basketball every day. He tried out for track and was doing well but couldn't compete due to his grades.I will have to pay his fines around $500 and I pay for his gym membership. I want to help him but I don't know own how.Any suggestions even criticism I can take it. via /r/Parenting http://ift.tt/2qkpeKR