Connect the two via Firewire cable. Boot your son's computer into Target Disk Mode. Boot your computer from the Snow Leopard DVD:
Booting From An OS X Installer Disc
1. Insert OS X Installer Disc into the optical drive.
2. Restart the computer.
3. Immediately after the chime press and hold down the "C" key.
4. Release the key when the spinning gear below the dark gray Apple logo appears.
5. Wait for installer to finish loading.
Follow directions being sure to pick your son's drive as the target destination.