[Mothy]

Get an old Apple iPhone, have a cheap SIM installed and activated, Turn on the "Find my iPhone" function and leave for him to collect. From memory you can set it up so he won't know it's active and if he turns the phone on, you will be able to find it's last known position.

Have one permanently wired into my car - for a small fee per month for the SIM (which makes no calls) I have a car tracking device. Using the same principle, your only issue is making sure the battery if charged.