Great points above, OP.
How much does DS know about the reality of an investment banking career? ‘Fiercely competitive’ is an understatement, and the working conditions can be brutal.
I am in STEM. I assure you that students from the quantitative sciences (primarily degrees from Schools of Maths, Physics and Computer Science) with excellent degrees from top universities are highly competitive for entry level positions in investment banking, those with PhDs even more so. They are the ‘quants’.
DS needs a good degree from a very good programme, and genuine enthusiasm for it in order to thrive. The suggestions above are good in general. The MORSE programme at Warwick in particular is very good, but only makes sense if it appeals. Whether Economics, Joint Hons or STEM is not the main question. Excelling and also creating a strong CV is much more important. I think he should therefore do what he likes best.
Gently, much of the fixation YP have on investment banking is likely centred on salaries. But someone who is successful at one of the world’s major tech or consulting companies can be on mid to high six figures by age 40 with low seven figures (including stock options) in tech a realistic goal by age 50. The working conditions are more pleasant and the jobs might be more satisfying to many. Is that salary not sufficient?
I am not sure how investment banking came to dominate the imagination of YP.
None of this is personal to your DS! But I have watched many very talented students adjust their dreams for the financial sector downwards, because the competition is so brutal, while others get on with interesting well paid jobs. (This from a well connected School that does place a reasonable number of students in those highly sought after entry level financial positions)