What year - 10 or 11? I tend to find with non-academic students that panic sets in in year 11. I've have kids as late as february before their exams rewriting all of their coursework. The majority of students, however complacent towards schoolwork normally, find it hard not to get swept up in the pressure around GCSE time. I think it's the realisation that the end is nigh. I also point out frequently that if they intend to go on to college, they will have a holiday that lasts from mid-june until (usually) mid-september when they've finished!
I would also suggest that if you become worried you speak to his teachers. You might find that he does more than he lets on. This also means that any problems can be tackled early by working with school. Ring up and ask to see his head of year or his tutor.
There's often support avaliable to GCSE students - like revision sessions and coursework clubs which is worth asking about.
Hope this is of help. I am sure you aren't alone in this problem.
Finally, when I did my GCSEs, I had an agreed rate with my parents per grade - so much for A*, A, B, C, etc. Bribery, as amicissima says, is very successful with teenagers!