I agree with @Grimbelina .
You’ve got an insurmountable clash here really to me - a school whose ethos doesn’t deliver on what DS needs to excel in his studies (structure, accountability), but also a DS who has a clear career goal which doesn’t require him to necessarily apply himself in A levels (I.e. he’s not got the pressure to knuckle down to get into high ranking unis as some of his peers may be aiming for).
It doesn’t sound like that conflict is going to be overcome in the next term, year, two years etc. Even if DS starts to see value in applying himself for a levels, with the purpose of keeping his options open uni/career wise, he’s not going to do that in the current school environment.
Can you take him out for a coffee or walk - change of scenery can help, treating him in a more adult way etc. - and really talk through why you think he should move (money being bottom of the list for this purpose!), the opportunities he would have elsewhere etc. Empathise with his concerns on friends but balance that with the adult perspective that strong friendships will persist, but also friendships do change between 16-18, and then 18 onwards. And that by moving he may well meet other friends who are on a more similar path to him and have a lot in common with, (not heading off to Oxbridge or the Ivy League).
But also perhaps asking him to map out what he thinks/wants the next 5 years to look like - you said most people done enter his chosen career until their 20s, so what would he like to be doing between now and then, what’s important to him etc. How would moving help him with that, what would happen if he stayed and there’s no school support as soon as he says he’s not going to uni
Is his Dad involved? If so, would he help present a united front? Or any other trusted adult who would back you up so it’s not ‘just’ Mum he’s hearing?
You’re doing a great job to support him through this - 16 year olds do struggle to see the bigger picture, change is scary, friends are the most important thing in their life etc… but this is also an important life stage and could really set him up on a great path to his chosen career, without feeling unconfident, thick and unsupported because of school.