DS is 15, yr 10. He is failing at school.
As a (retired) teacher I would be very concerned about this.
He's not particularly mature.
There is a degree of individual variation, and boys tend to mature later than girls, because they complete puberty later. By age 16 though, boys usually manage to catch up.
I have responsibility for him.
You have responsibility to him.
He is a child, whatever he and you believe.
It isn't a question of what I believe. In fact I'm very much against believing things. But it is a fact that an adolescent is not a child. And treating them as if they are a child is not likely to help. There was an excellent TV programme that explored this. It's available on YouTube (in bits, for some reason). It's called "Wait until your teacher gets home".
And I have to act in what I believe to be his best interests
Well of course you do. Did you really think I was suggesting otherwise? As a parent, until your child reaches the school leaving age you have a duty to ensure they are educated. We may have to disagree on how to tackle the problem though. I really do recommend that TV programme.