I have a boy in Y10 who was working way below his ability in Y7 and Y8. It took a meeting with the Y8 HOY who had got reports from all his teachers as my son was saying that he'd like to move up a set and for the HOY to tell him that it is not just ability that gets you put in a particular set, but attitude and behaviour.
That meeting seemed to be the thing that began to motivate him. As concerned, caring parents we had told him all the things the HOY mentioned in the meeting, but because we were just his parents he took no notice. It took a quite serious meeting for which we had prepared and we both took time off work and which my son attended for the penny to drop to him.
Result - changed person. I will say that he was very worried about being bullied (he had been in Y4 and, he now tells me that in Y7 some boys had been very mean to him) and so did not want to lose his place in the "cool gang" which was part of his reason for not giving his all academically.
He has now matured, too and attends any after school revision classes going - he's in Y10 - and is doing very well.
There is hope! But, you can't do his work for him. You have to find a way to get him to realise that and to realise that qualifications do matter very much in this day and age. He could leave school and do his qualifications as an adult at night school, etc, but he would have to pay for that. It's much easier if he gets them at school (another argument that seemed to work for us).
Good luck! Also, Y8 does seem to be a bit of a coasting year if you're that way inclined. I have a daughter in Y8 who is completely different and working very hard. It depends on the child's character.