I think you need to work out what his intrinsic motivation is and work on that. The future is too far away, but if you can build in ‘what do I want in six months time’ then work from there.
Eg
In six months I want to be able to apply for XYZ college to do ABC course, so I need ... grades.
And work out where his gaps in knowledge are- the problem with revision is everyone naturally revises what they’re good at. You need to get out past papers and work out what he doesn’t know- and this will be hard as he doesn’t know what he doesn’t know, as he doesn’t know it 🤣.
But apart from that, you need to let him be the driving force. Offering money as an incentive can actually demotivate children, as they interpret that as ‘you don’t think I can do it’ or ‘you think I’m too stupid to understand why the grades are important within themselves’. The reward is the door that open to the next stage.
And to help him picture that future, definitely take him along to university etc open days in September. If he can see where this is leading, that will help him see the point in what he’s doing now.
Do you have any older cousins/older children of your friends who perhaps used to go to his school and are now at university? Perhaps ask them to write/email him about what they’re doing now, and what grades they got, and how they’re enjoying their college/university course. This can be hugely motivating for someone who is struggling to see the point in working or revising for things that they don’t feel interested in.
Essentially, you want to set him off on a good trajectory right now, rather than allow this to spin him into a downward spiral.