When i was younger my dad had a subscription thing on my phone so that he could tell where abouts i was. But more importantly where i had left my phone... it was not through GPS so it only closed to position down to the neariest mobile mast but that was usually enough to work out where i had been and where i'd left it.
Mobile Defense app? He'll get a text initially when you establish a connection, but if you intercept it first, then the program will just run in the background.
Mobile defense works, there's also a gps family tracking app, costs about $30 on the market I think. Then again, you need to worry about your kid being smarter than you are with the phone and able to bypass it.
WaveSecure does the same thing... Only thing it'll show is that GPS is active, and if he has a task killer, it'll show up in there. They make it so that if you are rooted, you can "hard install" the app on the phone, making it impossible to delete it.