Be interested in other people's experience of this. DS is August born, in Y9 and has inattentive ADHD. By this stage with our oldest we were very confident they'd do well at GCSE, although their first year-to-go exams in Y10 were a little wobbly while they got used to how to revise and sit exams. But we knew they would learn from it and were probably going to be more or less the 'same kid' when they sat the exams.. They went on to get similar results to DH and I, eg at the high end, and are studying A-levels now.
DS is still on the less mature end of his cohort and basically we know he is 2-3 years behind his peers developmentally as is common with ADHD and will be sitting his exams at something of a disadvantage and it's as well he's as bright as he is despite his difficulties that he can compensate for that lag to some extent.
So we have just under 2.5 years to go and it could go in all sorts of directions. He might not develop much more maturity and have a lot of difficulty revising for and sitting exams, he might suddenly have a developmental spurt (as he does now and then) and be much more ready and able to cope and be able to do well, he might get massively hormal and turn into a sullen uncooperative so-and-so and decide he can't be bothered (at the moment he seems sweetly optimistic it'll be OK).
To be clear, I'm not demanding he gets top-notch results, it's just tricky because DH and I only have experience from our own families of 'Strong GCSEs, no problems getting to A-level, goes to university' and while I'm aware of other options and happy to investigate them (and we will) DH is finding this very hard to cope with and to dispell the idea that you will be significantly disadvantaged in choices going forward you don't get good GCSEs, which I don't think is the case.