You sound very caring and supportive and that is great for him. I can only echo the sensible advice of others and maybe just add one thought, which is, does he actually want to be at this school, studying these subjects? How solid is his ambition to study law? I was at a talk for year 12 parents recently and the one overriding thing which struck me was when the teacher presenting said, "this is your child's
application [to university] and they must own it." He stressed that children need to learn an awful lot about themselves and their interests and passions in order to get to a place where they are ready to apply to uni. He was very keen on gap years, partly for this reason - a lot of growing up to do.
Perhaps your son has doubts about the path he has selected (pre-GCSE)? Perhaps he isn't as interested in the subjects as he thought he would be? Maybe he is having doubts about Law? And maybe he doesn't really know what doesn't feel right, but something
doesn't? I am sure this isn't the case with you, but it is clear to me as both a parent and teacher that sometimes a parent has plans and desires for their child which the child then kind of takes on, and maybe finds hard to challenge, even in their own mind. I heard some parents saying to a colleague 'but she has always wanted to study x at y university' and the teacher very sensibly pointing out not only that 'she' didn't actually have the necessary GCSE grades, but also that 'always wanted to' is quite a dangerous idea in Year 12. A friend's son had 'always wanted to' study Medicine, and his school wanted him to as well. One day he refused, point blank, to get out of bed and go to school. He didn't return (he was 16). Two years later he went to college and is now very successful in another career entirely, nothing remotely connected to Science, let alone Medicine. He now says that he felt he suddenly had no choices and his life was mapped out for him, based on little more than an idea he had when he was about 8.
I am not suggesting any or all of this applies to your son, but it is probably a conversation worth having, as it might reveal something of what is underlying his current unhappiness. I can see you already have broached it, but maybe he finds it hard to acknowledge that he isn't coping and to think seriously about the alternatives. Sometimes it's easier to try and plod on than risk everything by admitting it isn't going to plan.
You seem so sensible - not pressurising him and being willing to think outside the box, that I am sure you will find a way through this. Best wishes to you both.