What do you do in the evenings? Is there a time when you are sitting working? That might motivate him.
Also, what about starting from the positive. I know that I find it hard to settle to work when it just seems too much. So what about sitting down with him to make a timetable, but start by listing his leisure/relaxation activities in two lists major (going out with mates, footie, swimming) and minor (listening to a favourite track/checking e-mails). Then split each day into three sections (morning/afternoon/evening). Before you even thing about revision, insist that he choses for each individual day one leisure slot. So on Thursday he might pick the evening cos he always goes out with his mates, and on Friday he might pick the morning cos he needs a lie in!). Even specify what leisure activity he will do during that session. He must not work during that time.
Then list his subjects in two lists: ones he likes, ones he doesn't. In each of the remaining sessions put two subjects, one from each list. Try to break them down too if possible - so don't just put maths, put maths-transformations (2 or 3 topics per session). He should be able to get the info for what to study from school or revision guides. Then when he puts his finished timetable up on his wall next to where he'll be working, he needs to put a poster next to it that says "Remember, never do more than 20 minutes without a break ... break ideas ..." and put down the list of minor leisure activities.
By focusing on the positive, and remembering that revision is very hard, he may well feel more able to cope with it.
As to how to revise ... there are lots of methods. Do you have the kind of family in which you could all discuss what does and doesn't work for you? Might help him to find his method. My favourite is the list and tick method - I write list after list of what to revise, look each thing up and write it in my own words on recipe cards, and then tick it off on the list. I got so good at this method that by my finals I was spending more time ticking than anything else (and therefore felt very good about myself!)