I did my 3rd year at Sciences-Po but this was 20+ years ago although I suspect it hasn't changed that much. However, all my courses were in French so quite different to your DS and that could be a fundamental difference. I attended lectures/seminars with French students along with a few internationals.
After an English university, it was brutal. Sciences-Po is VERY serious - no surprise given that (as you probably know) it's 2nd in the world for social sciences (after Harvard). I had to pass an entrance exam at my English university before I was able to apply. I then had to pass a further exam on arrival in Paris before I was admitted.
We had seminars Mon-Fri (and Sat mornings) that could be anytime between 8am and 10pm. There were mandatory. If you missed 3 seminars during the year (regardless of reason), you were out. You had to present to the group during 2 hour seminars and everyone was marked on oral participation (in addition to usual marking) so if you were presenting, it was usually a tough audience who gave you a real grilling as they fought for the limelight and those high scores. We had exams every 2nd Saturday afternoon.
The French students were incredibly serious and didn't interact with us international students unless it was in a seminar. I remember arriving at 8am one day to find out a seminar had been cancelled and one of my fellow students was pissed off because he'd lost his seat in the library.
Everyone had briefcases and quite a few of the male students wore suits. By Christmas, I was also carrying a briefcase and wearing a suit jacket (and jaunty scarf) with my jeans as a compromise.
Socially, the only effort made with international students was to hold a champagne soiree just before Christmas (so 3-4 months after we'd arrived...it would have been useful to do this earlier!). I did get invited to a professor's apartment for drinks along with the rest of my mainly French seminar group. And they did also hold an amazing party on the top floor of Beaubourg for all of Sciences-Po. Fabulous experience!
Of course things may well have moved on, and I suspect will be very different if the course your DS likes is all in English. My experience was very much being slotted into business as usual with the native French students.
It was a tough year. I grew up a lot. It definitely changed me. It felt more like a business school and when I returned to the UK for my final year, I was pretty much done with studying and itching to get working instead.
I've worked in a number of French companies since I graduated, or with French people in global companies (including being Paris based for a while) and being a Sciences-Po alumni gets me a lot of kudos with the French. No-one else really cares though. I don't know if I'd get the same kudos as a non-French speaker though.
In summary, it's hard and competitive. As you'd expect. But I imagine doing it in your native language will make it less hard.