I think the easiest thing you can do is set a password on the system and use remotecontrol to log your son off, which should require a password to get back on (there are ways around this, but I doubt your son would know of them). Along with that, you could use a hotkey program like AutoHotKey to enable a fullscreen image to tell your son whatever it is you want to tell him, and then a script to do the logoff. This will require you to learn up on how to setup scripts for this program, however.
Honestly, you will probably be putting in far more effort when instead one could just go speak to your son. But I understand parenting is not always so simple :/