a lack of skill. Make sure he understands he's driving around a 4000 lb battering ram. It WILL kill and maim people. That and the threat of him having to pay for citations and accidents should be enough for most kids to drive safely.
Some stop signs can be hard to see, but that's not an excuse. If you are driving in an unfamiliar neighborhood, every intersection should be treated as a stop until you are sure that isn't.
Intersections that aren't stops should be treated as yields because so many of us are distracted. Sure, the other person would be at fault, but at best a wreck will still ruin your day and cost you a lot of time getting your car repaired or replaced.