Why does an iPhone rename itself with (2)
That occurs when your Wifi network, or another Wifi network he's been connected to, think that there are two devices with the same name. It will then rename the device with a (2) or (3), etc. This is actually a bug in the software that Apple has used with the latest iteration of Yosemite and the Airport Extreme.

You can resolve this by shutting down your Airport Extreme, renaming your son's iPhone removing the (2), then turning the Airport Extreme back on.