The padlock icon simply means the particular network is password protected.
Look in System Preferences > Network and see if Airport shows as "Connected". If it does then he is connected to the WiFi network, but is not "seeing" the internet. You should be able to verify this by opening a Finder window then look in the sidebar under Devices to see if your MBP shows up (assuming your MBP has sharing enabled).
If he is connected to the WiFi network then logon to another account on his system and try connecting to the internet there. If he can then the problem is in our son's user account which cuts the solution space in half.
Let us know what you find.
