You don't need a second router. You need either a wifi extender or a powerline adapter. A wifi extender would be placed in a spot in your home closer to the outbuilding but still receiving a wifi signal from your router. A powerline adapter would be plugged into an outlet near your router and connected to your router, either by ethernet cable or by wifi. Another adapter would be plugged into an outlet in the outbuilding and connected to your son's device, either by wifi or ethernet cable.