This is the key difference in responses.
OP’s thread starting post asks: “HOW to get into the stock market/investments world?”
But then follows with: "Son wants to become a QUANT "
Followed by pointing out that the world of investment bankers is foreign to them so OP thinks son should take finance classes “so he’s not completely a fish out of water” doing an internship at GS/MS.
Quants do pursue various roles in finance/IB/PE/VC.
My interpretation as stated by OP that OP’s son wants to explore opportunities in “the stock market/investments world.”
There are great programs outside of MIT or any other college designed to introduce students & professionals to varied roles in finance/IB/etc. Most require just Excel skills & basic math while others require knowledge of Excel, calculus, & probability. No need to interrupt OP’s son’s planned courses for his major at MIT. The student can take the courses online and on-demand from almost anywhere in the world.
The overly simplistic advice of just calling alumni contacts involved in the industry does not recognize the intense competition for these type of positions–even for Wharton, Harvard, NYU, etc. type students.
OP wants his son to learn more about “the stock market/investments world” as do thousands of other students. There are many courses designed to do just that. Some are fairly basic while others are quite detailed for quants. I encourage OP & his son to search the internet for the dozens of readily available options.
There are entire websites devoted to this topic.
Whether OP’s son wants to explore “the stock market/ investments world” or wants detailed knowledge of what quants do in finance roles in the world of stock market/investments/IB/VC/PE, there are courses readily available.
P.S. My understanding is that posters on CC are not permitted to direct others to what may be competing websites and I do not want to appear to be promoting any possibly for profit websites so I will refrain from doing so, but I assure OP that there is substantial worthwhile material out there to satisfy any concerns.