Chinese pupils are assumed by schools to be brilliant at maths, but may hit problems when they get to university, either because the step up is tough, or because they discover they are not that interested in just studying maths.
Some pupils, particularly from those from certain ethnic backgrounds, do well at school mathematics by studying very hard and practising or learning every past question. This works on the current A levels which ask relatively easy and repetitive questions but clearly this strategy doesn't work so well at university, when the exam questions vary much more! It is noticeable that students from certain cultures struggle to make this transition.
(He still has the option to essentially graduate with a maths degree, albeit from the economics faculty.)
There is no university in the UK which is letting people graduate with maths degrees from economics faculties. High maths content, yes. Access to specific financial/economics maths graduate programmes, yes. Maths degree, no. The requisite pure maths content is missing.
Even in my day, when firsts in other faculties were as rare as hen's teeth, the maths students seemed to get either firsts or thirds. So a fear that taking more maths if you are not supremely gifted, runs the risk of working very hard and still doing badly.
The statistics don't support this. At any of the top universities in quantitative subjects 25-35% get firsts, 35-45% get 2:1s and the rest get 2:2s or thirds. I have not known a very hard working student get less than a 2:1. I have however known many students who found maths different and harder than they expected, so disengaged from the course and ended up with 2:2s and thirds.
Until he gets his first year results he can't work out if he is good enough to carry on with maths. He has found some bits tough, and some straightforward, but does not know where that places him.
This sounds normal. In my experience it is often the better students who find things tough, as they are trying to understand everything, while weaker students just accept that they don't understand some parts. In general most students find that they need the first year results to place themselves.