We lived in North Andover for ten years, so I’m more familiar with Brooks and have nothing but good things to say about it. It’s a lovely rural campus with a tight community. Our neighbor’s three daughters all rowed there (one of them and her husband now live and teach at Kent). I also worked in Concord (and our son was born at Emerson hospital), a quintessential New England town with lots of U.S. and literary history. Due to its location near town, I’d choose Middlesex, but that’s based on my own aesthetics* which have nothing to do with what your son might be looking for. I don’t think he can go wrong with either choice.
@Atria can give you student insight to Middlesex. I’m hoping she’s still checking in.
*I’m known for thinking there’s not a bad choice among any of these schools and that most decisions are made on hair-splitting unless things like size and location are deal breakers for you, but you know those things before applying. If I were the student, I’d make my decision solely on how lovely I think each campus is. The prettier the campus, the higher I’d rank it. So, you might not want to ask me to help you decide. 