Help! How do I locate a missing iPhone?
Sorry, but if Find My iPhone wasn't enabled on his iPhone at the time it was stolen, there's not much you can do to track it, as you can't remotely enable Find My iPhone.

Even if you could push an app to the phone, it's not possible to enter the iCloud account. Your option is to pursue legal means to compel the phone company to either track the device or terminate the service.