Wow, so many replies! Thank you.
To answer a few questions. Re the future career path - he really doesn't know yet. Teaching could definitely be a possibility, so the point about choosing a second language that's commonly taught is a very good one (the quirkier side of him quite fancies something more esoteric like Arabic or Czech, but that might not be the most sensible option). Otherwise, if not teaching, he still just needs more time to decide - and I sympathise, having had not a clue at his age. I'm not actually too fussed about the specific career aspect at this point - I'm a classicist myself, so I'm very familiar with the concept of people ending up in all sorts of interesting careers (and lucrative ones, if that's what motivates them), even though they've got a 'useless' degree.
In terms of subjects, he's doing IB not A levels - his highers are German, Latin and Medieval History. He also did French and Ancient Greek at GCSE, so plenty of varied language experience.
When I mentioned a 'semi ab initio' second language, sorry for confusion - I don't mean 'semi' in a formal sense, just that if he does French ab initio, it wouldn't actually be totally ab initio to him. Is that a problem at all? Does a university 'mind' if a student takes a second language ab initio when they're not actually a total beginner? I can imagine a potential benefit to him if it makes the first year a slightly easier ride (as someone who took up Greek at university, I know how tough that can be!).
I hadn't realised until we started looking that some universities wouldn't allow French as an ab initio language - annoying, as he could have continued French in the sixth form instead and then done German ab initio at university, but he just liked German a bit more (though his French was actually stronger!). Oxford doesn't seem to have it as an option, for instance - not sure he'll apply anyway, as I think he'd prefer a less literature heavy and more mixed course, and getting in would be a long shot - but still, the possibility that he might not necessarily be able to do German and French (as opposed to French and German) is something that we hadn't factored in.
Definitely wants to do a year abroad. I imagine he'd prefer a teaching/university based year, but would be good to keep options open.
Thanks for the other various bits of advice. As many of you say, next stop I think is for him to look really closely at the course modules - and perhaps for me to gently nudge him in the direction of French/Spanish or maybe Italian as a second language, rather than something a little less mainstream.