Sorry, long reply incoming @Arizona29
If you genuinely feel he is bright, but he was not able to get down on paper in the allotted time what he needed to, and so underperformed, then I am afraid you need to consider that he DOES have SEN needs.
In y9 a teacher casually referenced my DS’s autism at a parents evening, because her own boys were on the spectrum so she recognised it. We were left reeling… he did not have a diagnosis and no concerns had ever been raised. When I spoke to school they said they are not allowed to raise the possibility of SEN unless we raised it first. We were oblivious [despite having an older child with AuDHD and being ND friendly, so to speak... it presents completely differently from one child to another, one sex to another so we ‘missed it’]. When we asked why no-one had raised it with us when it was obviously impacting his in class performance and confidence, the Dty Head said ‘it never seemed the right time’.
Cue: frantic scramble to secure private assessments as NHS waiting lists would not allow us resolve his support and accommodation needs before GCSEs. He got extra time added to his exams very last minute [which sounds to be what your DS may need], but without additional support in class/lessons so it was still too late: he got 7 mediocre GCSEs and a BTEC but no place at 6th form as they required 7s.
He is now at a fab tech college, predicted A/A* in A Levels and off to uni interviews this weekend. But only because we had to sort out his assessments and move him to a 6th form setting that was geared towards kids who have an assortment of SEN needs (it’s not a remedial class, just a tech college that offers vocational/BTEC/apprentice courses with a tiny number of A Level and Access course pathways).
So, I think you need to sit with his HT and insist the SENCo is involved. You need to ascertain if they suspect there is a learning support need - autism, ADHD, dyslexia/dyspraxia etc. You need to consider whether you can fund getting the assessment done privately as you could have everything DS needs in place before he starts year 10. You also need to ascertain whether with that additional support/exam time he might remain on the main pathway, perhaps dropping one subject so that he could attend maths or English support - the latter was my DS’s issue, as was really understanding what was needed in exam questions - he, too, ‘knew the answers’ but was not understanding that it needed to be evidenced on the answer sheets.
And I would consider external tutoring in these topics/exam techniques as it positively impacts performance in all the other subjects. we did this for DS for Y12 only, and it helped him elevate his post GCSE grades from predicted AS levels of DCC to achieving AAB, so that he is now predicted A*A**A for A levels. The confidence he gained from great tutors really helped.
It cost us a fortune and we had to whack it on credit cards initially, but we felt he only had one shot at GCSEs. And my DH is still fuming that no one raised it with us in y7/8 when we might have got a referral via the GP and an appropriate support plan in school, but we have had to focus on where he is now. His self esteem was shot after his GCSE results and he didn’t speak to any of his class mates again after results day as he felt so humiliated (he’d been predicted 7-9s), though he has bumped into them recently and begun to open up, but only because he has made such progress.
I guess what I am saying is go into this asking - why do you think my bright boy is underperforming? What explanations might there be for why he is unable to get what is in his head onto the exam papers in the standard time allotted - and what assessments does he therefore need you and his dad to arrange? What do you see when you observe my boy in class (ie distractibility, focus, talkativeness, etc). Assuming you can get an assessment done and a diagnostic report highlights an issue, what options does your DS have in Y10/11? Could he potentially stay on a modified version of the main pathway, perhaps dropping a MFL for extra English (this was standard at my DSs school) or will they only consider the supported/reduced pathway. Happy to be DMd if you need.