Just buy the game, download and install, go to Steam folder , it should be either where you putted it, easiest way to find is to go to your steam library, right click propterties on any installed game, then to Local Files tab and Browse Local files, now that should be the game where is installed, you need to copy whole game, and paste it onto usb and transfer to your dad's pc.
Let him download files atleast for 5-10 minutes, then pause it, quit steam, then paste the game where the games is starting to install (you have to find where his steam folder is) and then Verify integrity of the game.
Note: This may or may not work, but in my case it did work for CS:GO
Note2: Steam games are located into steam folder under "Steamapps"-> Common-> and there you paste your game, note your game has to be the folder root, like Metro 2033-> inside shuold be files, you copy the "Metro 2033" folder to usb or whatever you want to use to transfer.