Ooof, the days of simple flight sims are long, long gone. There just aren't any, unless you want to install a very old game. Battlefield Bad Company 2 has rush mode maps that have very simple aircraft to fly, and it's quite fun, but there is often heavy competition to get them; and it's primarily a first-person shooter game, anyways.
Frankly, you'll probably have to install something very old if that's what he's looking for. I used to love playing the original Battlefield 1942 with the Desert Combat mod with a bunch of bots and just flying the aircraft around, raping stuff. You could set him up with that, because the AI never went for aircraft.
"VICTORYYYYYYY!" -Johnny Drama