Congrats to your son on his achievements and all his interests!
First, put it out of your mind that the college he attends can limit his career. His career will depend on him, not what college he attends. Do you think every person in the U.S. with a nice career went to a top 20 school? Not at all. Far from it.
Second, I think Clark is an excellent choice from among the ones you named. There will be a lot of students there like him. Another one to look into is Muhlenberg. American is great for political science. Oberlin is a nice school and has a very far left vibe as well, but it’s not in the northeast. If Ohio is a possibility, I’d also suggest Kenyon.
Northeastern is the one school on your list that a) would be a super-reach for him, and b) has a very different vibe from the other schools you named. It’s very preprofessional, not as much of a life-of-the-mind kind of place.
One more to consider: University of Vermont.