If he has time for only one museum, I'd recommend a quick visit to the Art Institute for a look at two really great icons of art, one French Impressionist and the other a slice of Americana:
A Sunday Afternoon on the Islanf of La Grande Jatte (George Seurat):
http://cgfa.sunsite.dk/seurat/seurat1.jpg
American Gothic (Grant Wood):
http://cgfa.sunsite.dk/w/wood2.jpg