sounds like he is working, don't be too hard on him.
also sounds like very poor exam technique, which schools will be working on quite hard now.
Also sounds like he doesn't quite have the maturity to do this on his own, so I suggest you help him.
Download all the specificatations for all his subjects - check at parents evening you know which exam board and which syllabus for every subject.
divide each subject up into a certain amount per week, and then subdivide those amount into two or three chunks to revise one day at a time.
draw up a timetable, with two or three subjects a day, and allocate those chunks to specific days.
allow plenty of time on the time table to review
Make sure the time table is finished weeks before the actual exams, so he can move on to constant exam paper practice.
For a maximum of TWO ( ideally only ONE topic per subject) - drop it altogether if he finds it too hard - some topics would take so much time to understand for some children that it is nt worth it.
Chemistry is a concern, because it is a very low grade, and most of the topics fit together, so it is difficult to leave one out without that impacting on understanding of the others. Chemistry needs to be worked through slowly and carefully, it is better to understand three quarters of it than rush through it and understand less.
maths and English have to be priorities.
Geography is a funny one, because it is easy to understand, but not revised very well on the whole, partly because students over estimate how easy it actually is.
i would seriously consider asking if he can drop French
Can you afford a tutor? Pick one or two subjects to prioritise with a weekly tutor if you can. I would suggest maths. That is likely to improve physics grade as well, in fact a maths tutor might be able to spend some time on physics calculations too.
Good luck.
Just remember, its important but not THAT important - if smeone can't actually do it, the most important thing for them to understand is that in the end, it is NOT the most important thing about them.
xx