GCSEs can be daunting as there is so much content to cover now. Good susggestions from @TeenToTwenties .
Exams start in just over seven weeks but that’s still time to make a difference to outcomes.
You’ve not given much detail @holiholiholidays so here’s what I would look at:
How is he doing in maths and English lang? Is he on target to get a grade 4 or ideally a 5? If not he should prioritise those.
Otherwise look at subjects he might take further – A levels, college – and focus on those, whether that’s science, history, PE, food tech, MFL.
It might be the case that he has to let a couple of subjects slide, and while that’s not ideal, depending on PGs and post-16 plans, that may be better if it means he can gain required grades in key subjects.
Having worked all that out, he can then be quite pragmatic. He should have covered most of the content by now, if not all (subject dependent for sure). So could he try a past paper in each subject he is focusing on (as there shouldn;t be any gaps in what has been covered), see how he does, mark it against the mark scheme (or can you support here? It’s important to be honest) and see where there are gaps in his knowledge. Then focus on those gaps.
There are lots of ways of revising but the important thing to do is targeting. So if he identifies he is weak on topic xxx in biology, make revision cards and get you to test him, use look cover check, make a mind map focused on that topic. Short bursts can be good – 20 mins on one topic then switch to another, then after an hour take a 15-min break.
If you can give more specifics (which subjects, what plans for after GCSEs) maybe we can help more.