can you download some past papres ofthe exam board internet site? Get him to sit one under exam conditions, then mark it with him. ( the answers are normally downloadable to) . Work out an exam timetable with him, say three 40 minute slots each day. Help him make a set of revision cards, each card has a question on one side, and the answer on the other., The answer could be a diagram, a definition, a set of figures, an example, These cards can be used to revise in short burst on car jurneys, in ad breaks, etc.
As you say, bbc bitesize is excellent. Is he doing the test bites, as well as the revicion?
You are doingthe right thing to insist. 15 year olds often just don't have the self disipline to do it for themselves, and just don't see the bigger picture.
Lots of praise and reward for effort...(even if progress is slow)
Incidently, studies have shown that the mosrt effective revision is done in the first 10 minutes of a recison session, and the last 10 minutes befor a planned break, so if he asks for a break, say YEs, in 10 minutes, these will then be a much more productive 10 minutes. If you just say yes straight away, you miss out that 10 minutes super boost!
In my experience children whose parents take an active interest like you are doing, in the end do consistantly better.
I know it's a drag, but good luck with it. ( I wish half my students had parents like you)