The feeling of panic when you first sit down with a pile of revision notes and books can be paralysing. The trick is to move past the initial fear, ignore the totality of the work and start organising the subjects into smaller and achievable blocks.
The first thing your son needs to do is to get a copy of the course syllabus. If his teacher has not provided this have a look online. Most public examination boards have their own websites and you can download topic lists and learning outcomes. You can use the former to create a master list of areas of learning and the latter to test knowledge.
Is he really going to be able to cover the entire Cultural Revolution in a night?
Go through the syllabus, dividing content into three columns marked "confident I know this", "need to go over it", and "not the faintest clue". Some boards will give weightings for various topics, which are extremely helpful when you are trying to prioritise. Teachers often hand out revision lists based on their experience of preparing students for public exams. If your son has one of these lists, compare it to the syllabus to try to get an impression of which topics need more time spent on them.
I would not discount study guides (if available) although these do tend to simplify knowledge and should be used as a support rather than a sole source of information.
After creating the topic list, your son should go through his notes, handouts and textbooks. If he is a typical student, these may well be in a state of complete chaos.
Don't allow him to become overwhelmed. Again, take a systematic approach and add three categories to your topic columns: "have all study notes", "need to get more information", "have no information".