All good answers here, and they're all correct.
However... that fact that a JW is dating at 15 in the first place speaks volumes to me about this boy, also, the fact that he's dating a non JW speaks even clearer. Either his parents dont know about him dating at all (most likely) because any JW parent would be absolutely livid at their 15yo dating a "worldly". Or his parents do know, if they do and they're fine with it then they're not strong in the religion and probably inactive but still call themselves JW.
This actually could be your opportunity to help this boy out of the cult. Be really nice and blow the JW stereotype of what non JWs are like. They're told that non JWs are greedy, lying, sinful, proud and quick to anger.