You will have to "unhide" your Router's SSID so that your son's Laptop can find it when he scans for "Available Wireless Networks". He then highlights that network and selects the option to "Connect". At that point, he will have to enter the WEP/WPA encryption key and should then connect to the network and have internet access.
A "profile" is saved so that his Laptop will connect automatically in future when that network is available. Also, there is a setting in the Properties of the Network Connection to connect even if the network isn't broadcasting, so this will have to be enabled if you set the router back to hiding its SSID.