I loaned him my car to practice with. I'm going to let him use my car for the next 18 months as well, but my rules were that he can't go places without at least one of: Husband Guy, Father Man or myself in the car with him (Mother Woman doesn't drive). He could take other passengers (he was only allowed two passengers before, a licensed supervisor and one extra).
He balked at that a bit and tried to convince me to let him drive places with his friends... he says he doesn't understand why this rule has to be imposed, but I think he does. He's just bucking at it to see if I'll move. The thing is, I trust him to use his good judgement, but people generally get stupid when they're with their friends. Adults do this all the time as well, it's not just teens. I explained it to him very politely and I stood my ground. I had to outright say "No" a few times, very gently, but to let him know that I really wasn't budging.
He took it well though. He said he'd follow my rules and cooperate with Father Man. I hope this means he really is growing up.