Mike - you mentioned in a comment to another answer that you still have the title. That's good in the sense that you can prove you still own the car. It's bad in that you're possibly liable for any costs associated with the car, like it being impounded as you also mentioned.
You have a signed agreement that he was to pay you by a certain date, and I'm assuming that date has passed. You might be able to get some money out of him.
However, legal documents signed by kids are problematic, especially if the kid is younger than 18 (the age of majority in most states). When someone 17 or younger signs a contract with no co-signature by a parent or guardian, then those agreements are difficult to enforce.
Sounds like you have a minor mess on your hands. Depending on the value of the car, you might just walk away from this having learned an expensive lesson. But, you should at least threaten legal action to see if you can get any $$$ from the kid, and small claims court might be an avenue to that end.
And, while it sucks that the car is impounded, the bright side is that it's out of the hands of kid and he can't do any damage to it, or get into a wreck with a car that is in your name, etc. The impound fee is far, far, cheaper than a multi-million dollar lawsuit from someone hit by the kid with your car.
Good luck.