You could phone up or go to a dealership or auto workshop and ask about your model of vehicle and the simplest way to disable it.

Example: on some vehicles, you can easily pull a plug/connector lead on the ignition control unit or engine management unit,often very easy and not very obvious. (Mechanics do this if turning engine over with spark plugs removed, so as not to damage the control unit)

Also similar can be done to starter motor circuit and as other Anons have mentioned,the fuel pump relay.

I wish you all the best in the difficult situation you find yourself in with your son.