My one-sentence summary of arc: minimalist common-lisp-flavored lisp-1 atop scheme. It takes quasiquoting and some other more dynamic design choices from common lisp and overlays them atop the more modern Racket runtime with as little code as possible. Other than that it's lots of tiny ergonomic conveniences. Maybe you should read/show him the tutorial next: http://ycombinator.com/arc/tut.txt. And arc.arc is a pretty fun read: https://github.com/arclanguage/anarki/blob/3fc14d16e9/arc.ar.... What really got me into arc was writing programs in one split window with the friggin compiler for my language in the other.
I did have a short look at the tutorial, but I had a bit of a hard time diffing it mentally, as I am still fairly new to Lisp. But I will definitely point him in that direction if he is interested in having more than a cursory glance.