Oh, bless him....Y10? I can see that level of stress happening in A2 year, quite a few people walking out of lessons crying & exhausted near the end, but christ, at 15, that's early.
There are loads of people I know who didn't get great grades and you'd never think it now because they're doing so well. Even if he 'only' gets 8 or so A*s, that's enough for Oxbridge, the LSE etc, isn't it?
Couldn't you persuade him that part of revising well is letting your brain have regular downtime so it can process things & let things fall into place, etc? E.g. when I did piano exams I'd have a disastrous practice session where nothing went right, tripping up all over the place, but then somehow the next day, miraculously, things had just sorted themselves out overnight and it was all in my muscle memory. His brain will carry on working whether he's pushing it or not. Also energy needs to be rationed out - don't work until you've got no more energy because you don't know when you'll have it again, whereas if you stop while you're still fine, not yawning uncontrollably etc, you'll be able to rely on it much more comfortably.
Could you take him out for the day this weekend? Depends if he'd be able to switch off and forget about home - hard to know in advance, would be awful if he was nervous the whole day and itching to get back. Maybe the cinema, that's good for distraction, tho would be better to get him out in the sun - tennis perhaps?
His inquisitive side will come back when he has more energy and headspace, don't worry about it. It's subconscious prioritisation! Form
Also this is the first time he'll have really revised, so he's still finding his revision feet, he'll develop a better technique as time goes on.
You sound like such a lovely mum. I have final year uni exams next week and my dad will be really angry with me if I don't get a 1st. He would never risk telling me it's ok to be a road sweeper!