As Bob Said
As Bob said, or at least alluded to, are you sure that this headset is advertised to work with the PS3? With game consoles it's not always as simple as just plugging in some random bit of hardware. If it doesn't say it supports the PS3 right on the box, I'd take it back and look for one that does. If it does say it supports the PS3 right on the box then maybe you got a defective unit or there's additional steps required that your grandson didn't take.