Mac OS X is a Unix-based operating system, and I'd like him to learn Unix.
I assume you're speaking from a concern of building a marketable skill set, based on this and other comments about coding. OS X uses a proprietary Unix kernel and tool set that has limited appeal to the business community. Linux is far more marketable, particularly Red Hat or SuSE.
Getting a good job is a long-term goal, but before that comes college. Looking at the computer science and engineering sites of a few schools, it appears that students are expected to quickly get comfortable with Unix programs and commands such as gcc, make, grep etc. A student who started a computer science program having only used Windows (and never using the command line under cygwin) would be at a disadvantage.
