New Drivers!

For those that have teenagers who started to drive, how nervous were you guys when they first stated to drive?

In our state (CA), new drivers can't have passengers under 20 in their car. My son is aware of the law, but something tells me he will be driving his friends and his girlfriends around. He's a new driver, license within the last 2 months and he doesn't have much driving experience. I told him I would like him to drive himself more the next few months to a year before he has friends in the car. I'll aways be worried if he has passengers in his car even when he's 50, but how else can I get through to him how serious this is?