I attended law school at a large university. In my view, that was the ideal LAC experience as law schools are typically small colleges in a large school setting. The best time for an intimate, exciting “LAC experience” is during graduate or professional school at a large university. This allows one to focus on academic material of interest to the student in a small, intense college while enjoying the benefits of a large university with respect to diversity, facilities, and social/intellectual options. My LAC experience was too limiting and suffocating (enrollment of about 2,200 to 2,400 undergrads in a rural setting).
As an aside: FWIW Middlebury College is my favorite LAC based on its size, location, and academic offerings so I do understand your son’s dilemma. I think that your son is approaching his decision in a wise & realistic manner. No wrong option/both are great options.