Tell him the only way out is through. The more he applies himself over the next two years the higher grades he will get, the higher the GCSE grades the more choice he gets for the next step ie A levels, some come with minimum grades like science is usually a 6, maths a 7 (but it should be an 8) because it is usually a one grade drop from GCSE to A level although maths is a 2 grade drop hence a 7 usually meaning a C/D at A level unless they work really hard. Obviously students do achieve A stars but they need a good work ethic and a good foundational knowledge to build on.
A level results determine which universities they can apply for and it may limit his choice on where he can go with lower grades. Entry grades are the lowest a university will take them in on, so AAA means a lot of applicants will have A stars. You can look now just for information. Ds1 started looking when he was in year 10 (and had a spreadsheet!) He just looked at the subjects he liked maths, physics, computer science, looked at the best unis for those and then what grades they wanted which included a 6 for English Language for some courses. You can then see that there are lots of subject combinations and much more choice.
All this is to say that the game is called best grades, learn what gets marks on papers, get the highest grades he can for his GCSEs. A levels are sometimes chosen for strategic reasons ie a university course requests it but hopefully it is a subject they love and that they are good at because again the game is get the highest grades. But also to understand that it might not be interesting content for GCSEs (mine never cared about Romeo and Juliet) but he still has to learn it and that any subject he dislikes he can drop and never do again in 2 years time.
Make flashcards, mindmaps, revision stuff as they go through year 10, this comes in handy for their tests in year 10 and also year 11. Lots of resources for free online, Youtube is fantastic for GCSE content just ensure it is the right exam board. This goes for revision tips from students and teachers plus covering the GCSE content. And keep asking for guidance on MN. It helped me when my first child was going through it and those who had gone before me could advise. Best of luck and don't underestimate the role of a parent, snack provider, testing their knowledge and watching films like any Austen so they can understand historical society which possibly ties into their English Lit poetry, oh and learn those poems now in year 10. Watch other films that depict war, All Quiet on the Western Front, Dunkirk, 1918 if they are doing Power and Conflict poetry.