Why am I not getting notified by email when my Find My iPhone is used?
You only get notified if you:

Play a sound on the remote device
Put the remote device on Lost mode
Ask to be notified when it comes back online if the device is offline

Just watching the location of the device on the app or iCloud does not trigger a notification.
There may be ways of finding out, they are discussed in this question:

However, your son could enable Status Bar Icon so that the device shows the location services icon in the status bar when the location tracking is active.
Settings → Privacy → Location Services → Find my iPhone → Status Bar Icon
This shows the normal location services icon (and not a special icon for Find my iPhone) if the device's location is being obtained through Find my iPhone.