@yellowmaple. Like @collegemom3717 I am curious, why these two schools? They seem so very different to me. Swarthmore is extremely liberal politically, very intense academically, and a bit nerdy. Wash U is more moderate politically, has Greek life, seems to offer more work/life balance, and the students seem more mainstream. The academic standard at both is very high. Either would be very challenging and would prepare your son well.
I have seen many parent and student comments on CC that report that the pre-med student culture at Wash U is very collaborative. I don’t have a point of comparison for Swarthmore. A search for “pre-med” in the Swarthmore and Wash U threads here might provide some additional information.
Good luck! They are both fine schools.