Unfortunately, Creative does not support the usage of the Zen Micro, or several of their other players, on a Mac. However, if your son has Mac OS X 10.4 (Panther) or 10.4 (Tiger), a freeware utility called XNJB should work. While you won't be able to use Creative's software, this program should get the job done.
If that method fails, try formatting the player on a PC (install the software and go through the setup process), then connect it to the Mac and try to use it like a mass storage drive. This solution was suggested by Creative for the MuVo so that it appears as a mini hard drive, but may also work with the Zen Micro. Granted, you won't be able to use the features provided in the software, but you'll still be able to load/remove music files from the device. (A method that is sometimes easier and faster.)
As a last resort, you can take Bredd's suggestion and purchase Virtual PC, which will allow you to run all PC-specific software on your Mac. However, Microsoft charges $129 for this, plus $99 every time you need to upgrade. Thus, if your only need for this software is to use the Zen Micro, it may be cheaper to purchase another player.
Hope this helps,
John