From your latest post it sounds as though he might be overwhelmed, willing but overwhelmed. I think in your shoes I would take a more active role, because of the situation in schools and because your own experiences will mean you could be of immense help to him - to ascertain the syllabuses and to get "pass notes" or the equivalents and to break things down for him for each subject - do piles across a table, one pile for each subject - so that he is clear with what he needs to learn, and also help him with things like trying to get his memory to work for him, aiming for "read it once and remember".
A lot of this is about technique, getting rid of the overwhelm, increasing memory confidence.
I really would encourage him to do all the subjects he is supposed to do but promise him you will help with each one (if you are willing to do this) and promise that the two of you will be able to get through it and it is worth it.
Motivation, determination, managing feelings - all these things are what get children through exams and also which stand them in good stead, throughout their adults lives - so the sooner they learn these skills the better. Hard graft is worth a lot more than innate ability.
I don't see this as pressure or stress (I went through something similar at 16) more letting him know that you have high expectations of him and for him and that you will provide the emotional and other support to help him.
It is also worth spending a day taking him to see various potential future environments - universities vs apprenticeships vs "no qual needed" jobs - so that you both get a clearer idea about what he really wants.
IME the only people who end up getting qualifications later in life are the ones with the motivation and determination and resilience early on, and so helping him with these skills now is only a good thing