A couple of suggestions:
Modern setting, Mantic's Walking Dead. Its faction v faction v zombies, easy to play. If your dad doesnt dig the zombies element just do faction v faction (Walking Dead Call to Arms comes out soon and focus's on this)
Near future, Terminator Genesis. This is more military vs terminators but if your dad doesnt like the terminators then it can easily be played as humans v humans. Alot of scope to use tanks or apache choppers. It also has many 'real world' weapons so there is that element of familiarity. i believe it is also discontinued and has been on clearance for awhile so probably going to be pretty cheap. Bonus if dad likes the terminator moofies
How about board games?
WW2 - Memoir 44. great game, easy to learn and fun to play, very light rules but also with some tactical depth. Great introductary game with many expansions. You can play it online as well to see if you like it.
Hope some of those helps