I nagged up to about Feb of Y11, when I realised it was destroying our relationship. So I had one 'chat' with him, where I basically said I would help and support, but I wouldn't nag any more as he knew what he had to do and why, and that ultimately it was up to him. So, from thereon in, as I walked past the back room he'd set up to 'study' in, if he was gaming, I'd just say 'You and I both know what you should be doing', and walk on by.
The school did what they could, putting him and 20 other boys into the Y10 'BLUB Club' 'Bright but Lazy Under-achieving Boys'
.
He got one grade point lower than he was predicted (as being capable of) in 6 of his GCSEs, getting B's instead of A's; so 2 x A, 6 x B, 2 x C.
He drifted into A levels, which by Feb of Y12 were also going downhill. So we decided that enough was enough, that what might amount to a couple of low grade A levels aren't worth the paper they're printed on (as has proven to be the case with other DC I know of). So we withdrew him (with 2 low grade AS levels) and, with him swallowing some pride, and overcoming some snobbery, he re-started at a Tech doing a BTEC in computing.
Finally a combination of being interested in what he was doing (he'd done computing before but without much commitment); maturity, and a different ethos, combined with me coolly informing him that I wasn't going to sponsor failure, that if he failed to achieve on this course, he was going to go out and get a full time job. Which I 100% meant, he knuckled down.
Anyway, he got 2 X Distinction stars and one Distinction; 160 UCAS points!
He's now in Y2 at a middle ranking ex-Poly uni doing software engineering. A course that asked for 128 UCAS points.
However, he is cognisant of the fact that many of his fellow students aren't that bright. He recognises that had he done better in Maths GCSE, and got a reasonable grade at Maths A level, he would be in a uni surrounded by smarter kids. But this is possibly only due to his subject choice, Computing, where there is a big divide between the ex-Poly entrance, and RG entrance, not much in between.
So, if I were you, at 14 I'd be nagging, so you at least feel you've done everything you could do.
I also recommend the Pomodoro time management technique!