OK, well I do know about TCP/IP, but not a lot about MACs. We can try a bit more if you like.

1. I assume that when both computers are plugged into the router, that your PC can access the internet just fine. Can you ping your Dad's Mac?

2. I assume when your Dad's Mac is plugged into the router, and your PC is not plugged in, that your Dad's Mac still can't ping the router.

3.Earlier you mentioned manually assigning DHCP and subnet mask to your Dad's Mac. This is not an accurate statement. DHCP means that the router assigns the IP address and subnet mask and default gateway to the computer. Usually when DHCP is selected in a computer, then the config fields for IP and subnet mask are greyed out.

It sounds like you are incorrectly configuring either the IP address or the subnet mask of your Dad's Mac.

3. Assuming we are OK on1 2, and 3, if you don't mind, do an ipconfig on your PC and tell me what internal IP address, subnet mask, and default gateway is being assigned to it by the router. Then tell me the same three things you have configured into your Dad's Mac. Of course, default gateway is just the internal IP address of the router. Usually the internal IP addresses for these setups start with 192.168.xxx.xxx, and the subnet masks is 255.255.255.0.