If he's not clever enough to go digging in the system settings, you could turn the volume down using the volume icon in the task bar, then hide the icon. (Hiding the icon is in the task bar settings.)

If he's clever enough to go digging in the system settings, then he's probably clever enough to disable any software you might install. In that case, your only solution is hardware modification: add a resistor either to the headphones or to the jack.