Unofficial discussion with his form tutor has revealed that the school would not legally be able to consider his parents wishes.
Well in that case - surely it's up to their son to write to his school and say he's dropping out? If he's taking adult decisions then he can't expect his parents to do it for him. First rule of adult life - don't expect other people to carry through his own decisions.
And yes, before he makes the decision he does need to consider his future options calmly and objectively, and he probably needs some help to do that. His parents can reasonably talk to him about what he will do to enter the military if he leaves school unqualified, or help him find out. Even if it's just telling him to talk to his school careers service before he makes his own mind up.
It is to being at an all boys private school with a very narrow outlook on life and knowing that throughout year 13 there he will be treated as a child even though he will legally be an adult.
If he hates school can he go to sixth form college instead? A lot of young people prefer the (slightly) greater independence and the more adult trappings of a sixth form college.
their plan of son getting highest grades at A level and going to top Uni are now not happening.
Well that part has nothing to do with deferring. That has more to do with parents who can't let go of their own ambitions for their son that don't relate to his actual character or preferences any more. He's simply reached that age. He's growing up and it's not all about what his parents want any more.
Blaming it all on decisions they took years ago is pretty pointless. No-one gets a re-run of life to see how it would have turned out. They are where they are now, they need to look ahead and accept that their child is growing. And accept that the harder they push him one way, the more likely he'll go another.