Does he want a light? I kinda hate having any encumbrances when I run. I even replaced my fraction-of-an-ounce digital watch with an even tinier watch because I did not like the weight.
Obviously, not everybody shares my prejudice, what with all the runners with iPods we see.
Soooo, with that out of the way, if I had to wear a headlight it would as small as possible like the Petzl Tikkina 2.
The Petzl Zipka 2 is even smaller, but I think with a string instead of a strap it would bounce right off my head while running.