I know you don’t want to add, but did your son consider Vassar? My daughter who is a learning-for-learning’s-sake-not-Greek-partying kind of kid really loved it. Alas, the financials did not work for us.
Of the schools you mention, the only one she visited in depth was Dickinson. Maybe she was just unlucky but she didn’t see a lot of student engagement in the two classes that she visited. I know that @intparent 's daughter had a wonderful experience there.
Good luck with your search!