I would go onto the school's website now. It should list the exam boards they use for each subject in KS4. (In England there are several different exam boards and schools choose which ones to use for each subject taught.)
Then google: 'maths gcse +specific exam board past papers' for example. (Maths, Science and Modern Foreign Languages are a bit complicated in England as they have different levels: either Foundation for those students working at a grade 5 or lower or Higher for those students looking at a 5-9 grade.) Take a look at the past papers of various subjects with your DS so you can see what's going to be expected of him in eight months time.
The past papers will also have the mark schemes.
I don't mean to scare you but GCSE mock exams are often in Nov/Dec so that's really soon. I guess it will give everyone an opportunity to see where he's sitting academically.
Does your DS have a heritage language? If so, it may be worth speaking to the school to see if he could sit a GCSE in that.
The school website should list the English literature GCSE set texts that they're studying. You will need to get him to read those asap. My DS is in Yr11 now and they've covered all of the set texts now apart from one. At my DS's school, a large part of Yr11 is revision in English lit and language though so providing he's read the texts there will be recapping/practise time which will help.
Maths he may also be in with a fighting chance as I'm assuming many mathematical concepts are universal. Maybe Science too?
There are also lots of good revision guides: My DS's school uses CGP revision books. You can buy them online.
My DS's school runs before and after school interventions in Yr11 both for pupils hoping for a 5 (which is a strong pass) and those aiming for 7-9 grade so hopefully your school will do too.
What is your DS's academic profile like? What sort of grades was he achieving? That will make a big difference in terms of what's going to be possible in eight months.
Does he have an idea of what he wants to do post-16? Again this will determine the best course of action in terms of GCSE advice.
Is the school in question private or state?
Finally, all this probably seems really overwhelming to your DS and you. The GCSE system is pretty brutal. Whatever happens, there will be post-16 options open to him if he doesn't get 'great' GCSE results. (He will have to re-take maths and english language if he doesn't get a 4 anyway.)
As @clary says, there are really knowledgeable, experienced, helpful people on this board (they are one of them!) who can help if you have specific questions about particular subjects once you know a bit more in the coming weeks.