I haven't read all the thread.
I live in Sheffield, studied at Sheffield University, taught at Sheffield University. SU is a Russell Group uni; we also have Sheffield Hallam uni, which,whilst not RG, is well thought of (I believe).
We appear have a LARGE number of Chinese and Singaporean students. I am told that some courses are now majority Chinese (don't know which, or if that's true). There has been a lot of Chinese money coming in to Sheffield Uni, and involved in building new student residences; some of these new residences are marketed specifically for the Chinese student community (I know this is true, the marketing literature is in Mandarin!) I know someone who is employed by the uni specifically to teach English to Chiese students who arrive a couple of months before their course starts, because some of them know so little English. There are signs in the university buildings in English/Cantonese/Mandarin advising new arrivals on how to cross the road safely, how to use English style toilets -
On the other hand, from the figures in the post above, the number of Chinese students is still less than 10% of the uni population. And I think that Spanish is a pretty unusual choice for these students (I have a Spanish degree). I think that studying Spanish will immediately mean that the OPs son will be mixing with more European students than on other STEM type courses.
And Sheffield is a multicultural city, with a (small) pre-existing Chinese population, and a choice of shops and restaurants for Chinese palates - you need to be able to cure homesickness at times!
OP, I wish your son good luck, he sounds a fortunate and interesting young man!