If you can get a static IP for your router from his and your ISP, without additional cost do it. That is a helpful thing in many cases where you want to reliably get to another computer and visa versa (you can use reverse connection too from his machine to yours if his firewall is persistantly a pain or you don’t know how to port forward and your network is open or properly configured for 5500 port forwarding).
However, if you can’t get the ISP/s to issue static IPs for free, then you can make life even more complicated yet solve this DHCP issue for your router with a public and often free service market segment like DynDNS.org. DynDNS has a little app/widget called dnsupdater you can download, and you can create an account with them (free for first 5 dynamic, and 5 static IPs) and use their service to register your father’s computer/network with dyndns.org, which in turn will be used to find your father’s system regardless of the IP address his ISP has assigned on a given minute (some of them expire every few hours or days and re-lease or reissue a new IP as a result from a pool).
There are a LOT of sites about this topic, as well as Google is your friend with search stings like “Dynamic IP VNC” or something like this.