What @MITChris said. If your son gets in he shouldn’t worry about the difficulty, but about the fit. Nobody gets admitted who can’t do the work. They do not guarantee a 5.0, but if you are reasonably diligent you will graduate on schedule. If someone is lazy, wants to be spoon fed, or doesn’t like STEM topics, another school may be a better fit. MIT expects students to take charge of their own lives. There are plenty of helpful resources around, but in general you are the master of your own domain. You can’t expect anyone to give you detailed instructions for life there - you must figure it out yourself. However, the culture is highly collaborative. A substantial portion of my education came from observation of and interaction with other students. The student body is one of MIT’s greatest resources. Friends I made there over 50 years ago are still friends today.
I found it to be and think it still is a unique university, but not because it’s “hard.” I suspect there are plenty of other places where the classwork is equally difficult if you are a STEM major. I think what makes it special is the dynamic atmosphere - the place never stops and many people are obsessed with making stuff. It really is mens et manus. Students are allowed great independence (in fact, it’s expected), and they get very revved up about what they are doing. It’s like visiting Manhattan: you can’t help but get caught up in it. Sure, ennui will creep in from time to time, but it’s not a good place for lazy people.
If he goes there your son WILL develop great problem solving skills. IMHO, that’s really MIT’s stock in trade.
Of course, your mileage may vary. My only other student experience was in law school at Harvard, which is totally different - dynamic in its own way, but nothing at all like MIT in rigor and workload, but that’s comparing nuts and mangoes.