Don't Go Small
This advice may seem old-fashioned now that we have cheap little cars with lots of safety features and tiny footprints. But the experts agree: get some sheet metal around your kid. He's going to make a mistake in the first couple of years he has this car, that's almost guaranteed, and knowing your kid, it's probably going to be a big, dumb one. So you're going to want to put him in a big dumb car that can absorb some of the impact of whatever doofus move he pulls. Little cars have plenty of safety features for practiced drivers; but for the newbie, nothing beats mass when you do the accident math. A low center of mass helps, too, so no SUVs. Something with the general weight and stature of a tank would be great.