Wearing his seatbelt saved my brother's life on two separate occasions.
The first time, he was in high school - 14? I think...a friend of his was 16 and had just gotten a car; she was giving him a ride to school, hit a patch of ice and didn't know what to do - she kept pressing the gas even though the tires were spinning. They grabbed traction, and the car shot out into the middle of traffic, causing a 7 car pile up. My brother was in the ICU for three weeks after the accident.
The 2nd time, we were at my best friend's 21st birthday party, and another friend wanted us to go pick him up because he didn't know how to get to my friend's house. I handed my brother my car keys, cause he had "only" had 2 or 3 beers, while I'd been matching my best friend shot-for-shot all night, including during the traditional "21 shots on your 21st birthday". He was going down an unlit country road, says he saw a dog in the road, and swerved to avoid it - ended up driving down into a ravine about 20-30 feet deep. My car was totaled.
: Fortunately, it had the automatic shoulder harness that's attached to the door, and dinged really obnoxiously if you ever unhooked it. He wasn't wearing the lap belt.