The Wireless Network Adapter in your son's computer should have software installed with which you can scan for "Available Networks". It should find the SSID (network name) of your router so that you can select it and then "connect" to it. Once connected, the little icon should get a "halo" to show you are connected to that network.
After connecting, the "Wireless Network Connection" (the wireless adapter) should get its IP address automatically from the router and you should have internet access.
<<< it is unable to connect I believe because the LAN is unplugged >>>
If the PC also has a "wired" network adapter installed - the "Local Area Connection" - and it isn't in use, you will get the "Network Cable Unplugged" indication. This doesn't affect the "wireless" connection. You can disable the "wired" adapter by right clicking the Local Area Connection and selecting Disable, which will remove the error indication.