You could:
- Swap the hard drive cable ends that go onto the mother board (EASY for your son to do this)
*OR*
- You could change the drive letters from Administrative Tools / Computer Management (another one for your son because this option might still be hidden)
*OR*
- You could change the boot drive in the BIOS as Reece suggested, except that anything that references "C" won't find your files if they are now on a different drive (and vice versa).