Most routers are not going to have specific rules you can set whereas a certain account/IP is allowed through. Basically routers you can buy everywhere atleast do port forwarding, but as SR said, its not done by default. Your best chance would be to get your router manual. If you do ipconfig /all
on your dads computer (at a dos command prompt), you will get his internal IP. Then what you would do is telnet or open a web browser to the router itself (usually 192.168.2.1, but it may be your dads IP and and 1 like x.x.x.1).
Hopefully he hasn't changed the password. But if you look in the manual or on the net, you can find the default router password. From there, you can configure a port to be forwarded to your dads computer, from the internet. If you can limit it by IP range, not common, would be a good idea to do so.
Now, install a remote administration server, wether it be a trojan, vnc, rdp, etc. Change the port it listens on to the port you forwarded from router.
Now goto http://www.whatismyip.com
and get your external IP from dad's ISP connection.
Now go to another computer, using the client for what you installed, enter the IP and port, and you should be able to connect.