It depends on the boy. If they really won't tell you what they want and you don't have a clue I would probably go with a gift card. Better that than something they really don't want or that is even a bit offensive. Don't give a teenage boy aftershave as a passive aggressive hint that they should shave and fellow adults of the world, please, please don't give teenagers pimple cream as a present!
Maybe an experience gift, like concert tickets would be good if you knew what sort of music they liked.