How are you connecting the PC? Wirelessly or directly to the modem, or via a router?
If you are using a router then power that down and the modem. Wait about 30 seconds then power the modem on first. Once the lights are stable on the modem then power the router back on.
Seems strange that your lappy works, but your sons PC isn't. Have you tried connecting your lappy again?
You can login to the modem config by going to
http://192.168.100.1/ and entering the username & pass as root, but this will only give you information relative to the modem and not anything to do with connection problems.