Welcome to the Board!
Well, you have lots of options.
You can just open it, make your changes, then save and close it and send it back to him. That's the way probably 90% of corporate office users do it.
You could use Sharepoint, Groove, Office Live, etc. to share it between you, which allow updating a common file.
If you have a network you can create a Shared Workbook in which you can both collaborate.
You could use file sharing site, but that becomes more cumbersome, because it's notnecessarily shared.
Like I said, lots of options to consider, and no one is necessarily better than the other - it all depends on your needs and resources.
HTH,