Is it in a place he can reach to groom himself?

If it's been there for a while, I trust you're in the process of taking him
to the vet for diagnosis?  That is the only way to know for sure what it is
and to have it treated if need be.

First, what is "a while"?  A weeek, a month?

If it's just bald, not crusty, not red at all, not inflamed or
otherwise looking "abnormal" it may be the site of a previous injury.
Sometimes, especially when wounds get infected, all of the hair around
the wound falls out - the skin usually looks smooth.  It takes a while
for it to grow back.  If this is the case *I* would just keep an eye on
it to make sure the hair is starting to grow in.  If the spot is
crusty, flaky, or red or oozing at all, *I* would take him to a vet.
If it's been more than 2 weeks with no hair re-growth (at least
stubble), *I* would take the cat to the vet as well.

-L.

did you find out what it was?