You are describing my son.
For many reasons I took him out of school in Y8. It then transpired he was way out of his depth academically. Primary had over-stated his academic achievements, so he was actually working at a lower level in Y8 than primary claimed at the end of Y6. It was an unholy mess and I spent much of the 18 months I was homeschooling just filling in the gaps and strengthening the basics.
While he was still in school I was getting phone calls every day about his behaviour and he would frequently climb the fence at school and run off. But he would come home, curl up with his blanket and put fairly childish DVDs on.
It was like two different people. A teenage mutant ninja monster at school, but a much younger child at home.
Aged 20 he was diagnosed with ASD and put on a waiting list for an ADHD assessment (23 now and still waiting).
I understand why posters on here are suggesting tough measures, but frankly I could have implemented all the toughest measures in the world, but it wouldn't have changed the situation. He was out of his depth, not coping and very unhappy in school.
As I have said many times on here, he has no GCSE's but is employed, had been with his employer for over 2years, has his own responsibilities, had small promotions. His manager had a troubled youth and has no qualifications either, and that hasn't held his manager back.
Obviously it's better to have qualifications but it isn't the end of civilization if someone leaves school without any.