I studied Russian ab initio at university (alongside French which I had done at A-level).
As a PP said, you study the literature and history as well as the language, so it makes sense to choose a country that you're interested in generally. And I do think being able to spend time in the country is absolutely essential, so that would rule out Russian for me now.
Russian is a beautiful language, but learning it ab initio at university is really difficult, and to be brutally honest, I probably would have found it easier, more fun and more useful (in my personal life) to learn Spanish.
Realistically, very few Russian graduates will use it in their career - so from that point of view, Mandarin would be more useful. But I'd guess it would be even more difficult to learn than Russian. I would only recommend it if you son has a strong interest in Chinese culture and is really motivated to learn the language. Otherwise, if he just wants something that looks good on his CV, I'd look at another language or a different subject entirely to do alongside German.
How about Polish? There are a lot of Polish speakers in the UK, it could be helpful for some jobs here!