Deleting a user account on the PlayStation 3

If he is concerned about the data, he can do a restore on the PS3. It will wipe all data and it will be like the device is out of box. The new owner will have to set up a new account on the device for their purposes.

The option is available through the Settings -> System Settings -> Restore PS3.

Deleting an account on a PlayStation 3 does not directly affect your PSN account; your son's PlayStation 4 account will still work exactly as it did, before.

There is one caveat; if your PlayStation 3 is offline, and you have any account data that still needs to be uploaded to the PlayStation Network (such as data pertaining to trophies), this will naturally be lost.