If you and your family don`t have a problem with Steam you should try out Civilization 5. It is a great game (after all the DLCs and expansions were out) where you can spend hours and hours in just one match-up in multiplayer.

I'd also suggest <i>Terraria</i> (Softcore difficulty, of course) -- though if ease of multiplayer setup is a concern, I'd suggest (sadly) that you get it on Steam.

<i>Din's Curse</i> is a procedurally-generated Diablo-like that apparently has no hard upper limit to the number of people that can play in a co-op match. <i>Drox Operative</i>, by the same developer, also has multiplayer, and I'd imagine it works similarly.

I never played Risk of Rain in co-op, but it's supposed to be highly entertaining. Have your dad play Engineer, that's the easiest class .

even better

http://store.steampowered.com/app/427730/?snr=1_7_15__13

What about the Jackbox Party Pack?