OP, a few nuggets from my own experience. I studied Latin and French at school (passing up the chance to try German), read French and Spanish (the Spanish ab initio) at University, I travelled a lot in S and Central America as a student, I learned Portuguese privately then went to work in Brazil for a while and I now work in Asia where I have done some Mandarin classes and travel frequently to Japan.
I went to about 2 night school classes in Russian but never pursued it; however a school friend of mine went on to read it at University and now has a civil service job so vague that we are fairly sure she is a spy.
As others have said, learning Spanish is a walk in the park if you know French grammar so it would be an easy GCSE to bank but a waste of a chance to get access to full time language teaching in a school environment.
My good friend is fluent in German after studying it at University and living there for a while. She is high up in corporate comms at a multinational pharmaceutical company but she only uses the German sporadically because it is never the common language in an international context. Her German collegeagues are impressed at her skill and now and again she can eavesdrop without people knowing but it's basically a novelty.
Your point about the Chinese embarking on huge infrastructure projects in Africa is spot on. I come across a lot of Mainland Chinese Mandarin speakers in my work. Some young professional Chinese have hugely impressive near-fluent English learned without ever setting foot outside China. However there are just as many who can't speak a word and will always rely on interpreters (it's not just a case of "the young speak English and the older don't so eventually they all will", it 's a bit more random than that). It's no longer considered that unusual for a foreigner to speak Mandarin well enough to do business and many do so extremely well, reporting that they'd be utterly excluded if they didn't - even the English-speaking Chinese don't behave like the Germans or the Scandinavians and will revert to Mandarin at any opportunity. However to really speak it well you have to have lived in the PRC and your DS may find that when competing for jobs etc in the future he is up against a whole generation of US/Canadian/British born Chinese who speak both languages to native standard. They are the ones with the best future opportunities at the moment and they are returning to the East in droves after University.
It is a very tonal language (4 tones) and many words are identical except for the tone. The Chinese are not good at working out what you were trying to say if you get the tone wrong. If you say the Chinese word for "house" in the wrong tone, even if the vowels and consonants are pronounced perfectly, you may as well have said "combine harvester". It sounds that different to them. Therefore it is not a language that is easy to learn if you are not outgoing in class and willing to make all sorts of weird noises until you get it right. There's also a danger that if the teacher doesn't correct you every single time you will labour on under the illusion that you are getting it right when you are not.
As others have said, the concept of the characters shouldn't be too off putting though because the teaching methods for the characters at GCSE will have been devised carefully and the grammar and vocabulary will be taught in pinyin (i.e. transliterated using our characters, with diacritic marks to indicate the tones). Interestingly, many eduated Chinese can also write their own language in pinyin these days. They use it a lot for text messaging - input in pinyin and then they get a drop down menu that allows them to select the right Chinese character.
Grammar-wise, it's good mental exercise to learn a system that differs so funamentally from a romance language. Mandarin is easier in some respects than many European languages as no genders, cases etc, but of course has its own mind-boggling weirdness such as literally hundreds of words to learn for "some", practically a different one for every noun it seemed to me in my limited experience.
I suggest that your DS maybe watch a few Mandarin films with subtitles to get a feel for the rhythms of the language; they speak it in Taiwan and there are a lot of light Taiwanese teenage films to choose from.
Others are better placed than me to comment on Japanese but it is an amazing country and the language is not at all tonal, so much easier to communicate from the word go as a learner. The Japanese generally have very poor English speaking ability so the ability to speak Japanese is a must if you want to live there (although you can manage fine without it on holiday). However it's not as important internationally as it once was.
Finally swedishEdith I agree that Portuguese sounds a bit Russian, but that's only Portuguese Portuguese. Brazilian Portuguese is much more "open" and easier to learn and understand; I'd thoroughly recommend listening to some Brazilian music like Daniela Mercury for a taster.
Thanks Op for a really thought-provoking question and I'd love to hear what your DS eventually chooses. However I did have to chuckle a bit at your comment
"I would be dismayed at the thought of an English teen not having the opportunity to study French literature in the original at least from GCSE level." I loved French but God I hated the literature when I was 16 - La Porte Etroite is possibly the dullest book ever written and don't get me started on Le Rouge et le Noir. I'm not sure many teens would sare your dismay 
.