If the bare skin looks healthy, it's probably not feline mange
(scabies).  It might be ringworm, but that usually appears on the cat's
head.  Here's a website on causes of hair loss in cats.

http://www.peteducation.com/article.cfm?cls=1&cat=1338&articleid=195

Another possibility is that he got a cut on his back that's healed, but
the hair hasn't grown back yet.  Our cat, Phred, scraped his tail a few
months ago, and although the cut healed quickly, it took a long time
for the hair to grow back.  Phred's missing hair patch was much larger
than his cut because he licked it so much.
digbklyn