This was the answer I got from "DAD" @ Yahoo Answers:
From "DAD"
your son probably just configured the ip settings according to the network used on the LAN (not land) Party manually.

Usually, routers used at home use DHCP which gives your computer an ip address automatically.

Just right click the little icon that shows the network connection in the task bar and select "status". Then click on properties.
Select the "TCP/IP" option in the list and click again on properties.
There select "automatically" on the ip and the DNS Server.
After clicking ok, check if it works.

And on the Gateway question.
You use a specific "subnet" on your netowork. That is a range of ip addresses used on the network. This subnet has a unique subnetmask for all clients on this network. The Gateway address is the device that connects your network to another, here the internet. So the Gateway address must be the same on all computers used in your network. Not to communicate with each other, but to connect to the internet.

Each IP Address can only be used once on your network. So if your client has for example 192.168.0.10, your other computer could use 192.168.0.11.

This answer resolved my problem! Thanks!