My parents did the same thing for my brother, and here we are 5 yrs. later, almost, and the car is broken down, sitting in my parents drive-way, unable to be driven. It's in "storage" according to the insurance, so it's cheaper. He barely makes the payments to them, often times missing the payments, so that my parents have to make up the difference. It's been a headache since day 1.
Now for your situation, if the car is in his name, he has the right to take the car, once he's 18, if he so chooses. It doesn't matter if you're a co-signer, that doesn't have anything to do with the actual ownership of the car. So even if you report him to the police, it doesn't matter. The car is his. I know you are uneasy about letting him take the trip, but he's going to do what he wants once he turns 18. And legally, he can. All you can do is patiently and calmly explain to him how you feel about him going, that you have an uneasy feeling about it and that you're worried about him. Then you need to let him make his own decisions. That's part of entering the adult world, no matter how much, we as parents, hate the idea. You're going to have to loosen the rope, just a bit, and let him explore things for himself, and let him make his own mistakes. It's all a part of becoming a responsible, mature adult. I'm sure there are things you butted heads with your parents about to, at his age. And in retrospect, you might think, man I wish I'd had listened to my parents. And he might think the same thing some day. But that's his mistake to make, ya know? Just pray for him, that he goes in God's hands. Sometimes that's all we can do.