He must be up to some serious naughtiness for you to consider doing such a thing Fish, and why doesn’t he have a mobile phone? That’s almost unprecedented these days.
There’s such a lot to consider before carrying out your plan, such as a life long loss of trust if he finds the device. You must be terribly worried about him and I’m in no position to preach, but have you considered going old school and sitting him down for a good old heart to heart, but you’ve probably done that already haven’t you? I hope it all turns out well for you Fish.