The Parental Controls in Leopard are supposed to be rather robust. Under Leopard (10.5), the Parental Controls may allow application-specific restriction, I don't know. If so, you could just use Leopard's Parental Controls to deactivate the DVD player application.
If Leopard doesn't (or if you're not running Leopard), you could just create a non-admin account for your son to use and delete the DVD player application from the Mac. As a non-admin user, he couldn't install any applications, so he wouldn't be able to play a DVD.
Doing it this way, as opposed to removing or deactivating the drive somehow, would ensure that the DVD drive would still be useable as a means of reading data or backing up data.