The router's password is stored on itself. You'll have to either guess it, or use the reset hole/button on the back to clear its settings (which will be easy to notice).
You could also do something like installing a small keylogger in your machine and ask him to access the router because "the Internet is not working", but I'd suggest you explain your father how connectability status doesn't really matter when it comes to getting infected, except on exceptional cases (a threat taking advantage of a security hole you either haven't applied the patch for, or for which there is no patch).
Furthermore, normally you should only need to forward one port (preferably between 49152 and 65535) to be connectable.
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