Hello, OP -
I am a former STEM admissions tutor. I think you have great advice above, particularly from @MintoTime . A couple of thoughts:
C1 fluency is excellent; if the CCE isn’t accepted, this generally equates to an IELTS score in the region of 7.0-8.0, which is ample for academic study.
DS’ maths also sounds very strong. He is likely to be well prepared in practice for English universities. However the French maths curriculum is quite different to the English and IMO it isn’t likely to vibe well with the ESAT. I second the PP who suggest dedicated ESAT tuition if DS will be taking it.
I know next to nothing about career pathways in Automotive Engineering. I thought PP who raised the question of whether to specialise at the UG level, or go for an Engineering degree offering scope for more flexible employment followed by an MSc in AE, made an excellent point. I would pay a lot of attention to the Employability statistics in the league tables, which show graduate level employment by UG degree programme.
Where does DS see himself working? At what age? Might a 20 or 21 year old have a more difficult time gaining employment (which will never be proved)?
Also - and this is tricky because DS is clearly academically able - as an admissions tutor and a personal tutor I think @phyllidafosset ‘s suggestion that DS consider doing A levels, perhaps on a condensed timetable, has a lot of merit. I might see if he could place out of A level maths (and it useful to know if he cannot) and just do Further Maths and Physics.
This would acculturate him to British pedagogy and ways of living, and bring him up to drinking age, before beginning university.
My perspective on this is that I am in an excellent School, and my youngest personal tutees inevitably have some extra struggles. Most Engineering programmes in the UK will involve significant group work. Social confidence is very helpful.
Best wishes to DS