I agree, @Midwestmomofboys , Denison is much more diverse than it used to be and more than those you listed. As is Duke. And Williams. And even Trinity has been working hard at this. All have a preppy element, though. I didn’t think OP was looking for homogeneous as much as a place where there was a strong preppy element. And Greek life (which eliminated many of the really preppy schools that immediately came to mind.) But if it’s one-note he/she wants, many of the schools mentioned above won’t fit the bill, including Denison. Is your son in a frat and how big is Greek life in the overall social scene?
Btw, most of the kids I would define as preppy do not identify as Republicans (even the VV wearing, lax playing ones) so I would not include that in my definition. But that may be a Northeastern bias which others, including OP, may not share.