It sounds like you are sending him just the Xcode project file, but you will need all the source files.
If you assume your project is in:
/Users/me/Projects/MyDeadGoodProject/
MyDeadGoodProject (folder with source)
MyDeadGoodProject.xcodeproj (project bundle)
(Which is fairly standard)
Then you need to send the whole of the top-level project directory structure.
This is best done by zipping up the whole lot, which can be done from Finder by right-clicking and selecting "Compress MyDeadGoodProject" or from the command line (Terminal.app) using zip
:
$ cd ~/Projects
$ zip -r9 MyDeadGoodProject.zip MyDeadGoodProject
And then e-mail him the .zip file.
If you co-operate with someone else on a regular basic then it's best to get some central git hosting going, as that will be much easier to manage.