Good post @titchy - no one has said your DS is stupid @winterrabbit. I agree with titchy that actually, having ADHD does not mean he is not smart, but yes, it might well mean that A levels would be a challenge.
You keep saying OP that you didn't ask for advice about A levels and BTECs but tbf to me and everyone, your Op said this:
If he has done badly then what options are there for him?
which I took to mean If he has not done very well in his GCSEs, what can he do post-16?
Apologies if that's not what you meant. But I still think you and he should look at it. You and he will know next month how he has done - and I really think it would be a good idea to look at all the options before then. If you don't need them - happy days. Just a bit of wasted time.
Hopefully we have variously answered your resit questions - you have to resit maths and Eng lang if you don't get a 4 (a 4 in Eng lit will do tho); you can resit anything else but you will probably have to pay. I know a lad that didn't do well enough to get in for sixth form and he resat the whole of year 11. No idea if they had to pay exam fees tho.
Can I reiterate that BTEC level 3 qualifications are rigorous and not easy; they just suit different learning styles. Mate of DS's did a L3 BTEC in sport science and ended up studying it at Lboro - probably one of the hardest unis to get into for that subject.
My DD did well at GCSE, not very well at A level and then did a degree with only continuous assessment, no exams, as that's often where she crashes - and she totally flew. DS2 otoh does better in a timed, pressured exam. Different things suit different people. Best to play to your strengths as far as you can.