I really depends on the subject. Different subjects lend themselves to different types of revision. Some subjects are based more on what you can remember, others are based on logic, cause/consequence, etc.
Also, different children have different ways of learning things - some are visual and will make their own "mental image" of, say, a diagram or mind-map which they can remember as a picture, others are more "hard wired" to remember words/numbers, etc rather than mental pictures.
There's also different aspects of revision, i.e. one part is the remembering of facts, equations, etc. Another aspect is practising questions, i.e. past papers, practice question booklets, etc., to get acclimatised to exams and also how to answer different types of exam questions - it's no use remembering everything if you run out of time in the exam or don't know how to apply the facts you've remembered to a different style of exam question.
I think to a large extent, they have to work it out for themselves. Try different approaches, see what works and what doesn't. He's only in year 8, so he's a couple of years to work out what works and what doesn't before things get serious. Once he's done the exams and got them marked, he needs to concentrate on the feedback etc and self-evaluate what worked, what didn't, etc and learn from it.
Having said all that, however he revises, it's no substitute for not doing the work throughout the year, i.e. learning for tests, doing the homework, concentrating in class, etc. Revision (however he does it) isn't a substitute if he's not done his best throughout the year as it won't fill the gaps he's not done - revision is about reminding yourself of what you previously knew and could do, it's not about learning new things!