Thanks again for the replies. Shame the site was down this morning so I'm just going through them and have booked some time with DS later to discuss. He was up for this, which is good.
To answer some of your questions:
Natural Sciences - interesting, we'll look into that.
We've done the 'Which University' tool and there wasn't anything new on there, which isn't really surprising, which is what I meant by 'clutching at straws'.
All the additional subjects you've mentioned will be put on a list and checked out.
He's been to a couple of open days for Marine Biology. He loves diving and is very knowledgeable about coral reefs. Not so keen on studying plankton under a microscope or spending his days in academia or in a lab. Good to know but it kind of removes this option.
He's been to Imperial, Loughborough and Bristol for Engineering. Has written off everything except Mechanical and Biomedical Engineering, though has only been to the talk for that at Loughborough. Might be worth checking out the Imperial course and maybe visiting KCL for that one. Thanks to those that suggested it.
He has mentioned a gap year, but I'm not sure how useful it would be in terms of helping him decide what to do, unless it's very targeted. Also told him he'd have to largely plan and organise it himself (isn't that the idea?) He admits he's lazy. 6 days a week in school plus laziness mean he doesn't do much in terms of supercurricular activities. Gap year might be good to help him grow up a bit and become more independent. I'm not totally against it.
He doesn't want to look at Oxbridge. Says it's too privileged and pressurised (see also laziness, above).
Some of you ask some very good questions. I will put those to him. Sometimes advice or questions from someone who isn't a parent get a better reaction.
Again, very helpful replies, thanks to all of you. I really appreciate the time you've taken. We are very proud of him. What he could achieve if he gave 100% doesn't bear thinking about. He's been through a lot and is a smart young man, if very quiet, and has good instincts around people. Wish me luck.