Here is a good solution if you have what it takes to do it:
1) Go to any laptop or PC that is connected to that network. Maybe your dad's laptop himself (you can't? then ignore the rest you have no hope unless you reset the router. You can? Keep following the steps).
2) Right click the internet icon in the notification bar at the bottom right and click "Open network and sharing center" (My laptop is windows 8 but it should be similar for windows 7 too).
3) Under the connected network tab. Just left click the network he is connected to and a new small window should appear with sent and received numbers.
4) You will then see a button in the middle called "Wireless properties". Click that.
5) A new window will appear. Look for a tab on top called "Security" and click it.
6) Look and click for "Show characters" tickbox and Voila you can see his password and use it on your own laptop.
Of course, as I mentioned before, it all depends if you have access to a laptop or PC that is connected to such network.
Good luck on your mission