1. Insert the grey install DVD and restart the iMac.
2. Hold down the C key during start up.
3. Do an "Erase and Install".
This will clear everything off and leave you with a factory feel to the iMac.
(If you had banking data on there, and for example, would be concerned if you son has housemates that may have access to the computer, you use can use Apple Disk Utility on the Install Disk (select from either the Menu Bar or options buttons (dependent on OS X version)) and choose to erase the drive and Zero all Data. This takes some time but will mean that the data cannot be easily recovered. On Panther/Tiger install disk you can 7-way write random data - which is more at the erasing-government-secrets type level of security.)