Basildonbond - I reached the same conclusion back in yr 9 - mine is also 15, but in yr 10 now - he's one of the older ones in the year thank God!
It got to the point where I was sick of the sound of my own voice, and the H1 detentions for homework - it clearly wasn't working. No matter how much I nagged, it had no effect, neither did taking stuff away.
I don't know if it is coincidental, but in the last year, I have seen a sea change in him since I laid off. I made it clear that his homework was his concern, and that if he wanted to do whatever he wanted to do after he left school it was up to him - but he would need qualifications - and he seemed to switch on.
Now, he is pretty self organised, gets on with it, and is getting good grades. Every now and then, I will ask him how his homework is going, and he rolls his eyes, and tells me it is under control. certainly, he is producing stuff, and doing well, so I allude to how well he is doing, and how mature he is etc.... he's now really motivated - mostly through maturity and school - rather than me.
I haven't completely left him to it - more that I am a resource. Am happy to read through essays, or to test him on stuff if he wants, or to suggest further reading if he's engaged with something... A few months ago, he asked me to test him on the reproductive system - that was a complete joy....
I don't know how, and I don't know why - one of the mysteries of teenage life - perhaps the moon was rising in saggitarius - suddenly he engaged with stuff. but certainly, stopping being on his case seemed to help - you can help and support, and you can lead a horse to water, but not make him drink.
Ach - he's my first teenager, so am fumbling in the dark meself!