If you want to donate your iMac to a good cause... ;-)
You can install Windows XP or Vista in bootcamp on your iMac and then use your iMac as a PC until you’re ready to switch over (upgrading your PC software during the next iteration to the Mac version).
Recommended upgrade...
In all seriousness it depends on what software they will be teaching.
In this one case he simply needs to conform.
They both work about equally well for 3D modeling (the meat of 3d animation). But there is much more market for PC trained 3D modelers as that is where all the good games are at.
BTW if you sons long term plans include a carrier in gaming he should seriously reconsider.
Every gamer kid wants to be a game programmer/artist/author.
The gaming companies know this and pay peanuts while requiring constant death marches.
Typical carriers are less then five years from fresh graduate to krispy burned out ex-game programmer.
Make sure he has a backup plan. Game artist/programmer is in the same category or carrier aspiration as rock/movie star.