Revision and exams are a really difficult challenge for many ADHD people. I've done completed multiple degrees, and still don't think I've mastered it. And other folks have said, everyone is different, so it's more about finding out what works for your son than general advice.... Apart from, perhaps, that "just try harder" is terrible advice for everyone. So I'm just going to speak about own experiences, but hopefully it can get you thinking about what might click with your son.
Getting started and sticking with it - Even if its something I enjoy, I can find it hard to get started and if it's something I'm anxious about or don't enjoy or have had a bad experience with, it can be almost impossible. **Suggestions **- Having someone help me start and stay with me (so I don't just give up and do something else) is really helpful. Depending on the task it might enough for them to just sit by (maybe doing their own focused work, as a good model) or to be there for me to ask questions and bounce stuff off every time i get stuck. Its also important to learn what time of day works best for the individual, my brain works much better in the morning and if I try and work on something in the evening its 10x harder.
Focus and Engagement - Even if I'm really motivated, and supported, I can read the same page ten times and not take it in. For me, reading sequentially through something and trying to remember it, even if take notes, is not enough to engage my brain. Suggestions: Stupid stuff like starting partway through a book section so I find it a bit confusing, then flick back and forth trying to understand it makes it more engaging and stops my brain tuning out. Similarly, instead of trying to memorize a bunch of information, I respond well to questions/problem solving - if I'm trying to prove a point to someone, suddenly I'll be great at skimming through textbooks to find the info I need. Learning about the different states of matter could be mind-numbing, but trying to explain to someone "why turning into either steam or ice could make a container of water explode" would get me thinking about a bunch of relevant topics.
Memorization and Notes - I spent a lot of school struggling to write notes, then never reading them. Since then I've found non-linear approaches much more effective: mind maps, brainstorming with post-its, even drawing pictures. Stuff like mind maps reward brains that jump from one idea to another, rather than being expected to work through an ordered list. Being asked to write down all the ideas that come to mind on "Why did the Civil War happen?" is more interesting than reading a chapter on someone else's answers. Even if you don't know anything about the subject, you can put down some vague ideas ("ugh, slavery? something about states rights?") and then quickly look up stuff about it to flesh it out. When it works well, I suddenly realize, "shit, i'm meant to be studying and making a mindmap, not getting distracted and reading 'the myth of the Lost Cause'... oh wait, this IS studying!"
tl;dr - what helps is unique, but helping your son get started, making studying interactive and conversational helps, and maybe try mind-maps and non-linear approaches. If he can find a way to become genuinely curious he will motivate himself!