Olin – no weed-out mentality
Rose Hulman also has already been suggested as no weed-out mentality
An alternate approach would be to go to Haverford for it’s 4+1 program with UPenn. Haverford is the polar opposite of weed-out, has great FA and is part of a consortium with other top schools. In contrast with the more common 3+2 programs, the 4+1 allows your son to graduate with his class and also get his engineering degree. Liberal arts classes can only help him with his writing and reasoning schools. Those are rare in engineers, relatively speaking, and make him more valuable – https://www.haverford.edu/engineering/41-program-university-pennsylvania
Dartmouth also offers a unique program that allows you to completely miss the weed-out issue – a 2+1+1+1. Two years in an undergraduate LAC with small supportive classes for the intense foundation courses, junior year at Dartmouth, senior year back at LAC to graduate with friends, return to Dartmouth one year to complete engineering.
https://engineering.dartmouth.edu/academics/undergraduate/dual/
Here are the undergraduate LACs that he could choose from–
Amherst
Bard
Bates
Bowdoin
Colby
Grinnell
Hamilton
Hobart & William Smith
Middlebury
Morehouse
Mount Holyoke
Pomona College
Simon's Rock
Skidmore
Spelman
St. Lawrence
Vassar
Wesleyan
Wheaton
Williams