Criminal Law Questions? Ask a Criminal Lawyer.
First of all, there's nothing that a juvenile court can do to you. You are an adult so the court doesn't have jurisdiction over you, so if you are called to testify on behalf of your ex's son, the worst thing that will happen to you is that the judge will give you a talking-to.
Technically, the police could charge you with allowing an unauthorized person to drive your car. Generally if they were going to charge you with that, they would have done so at the scene. And your letter is not enough to prove a DUI, so I doubt they will charge you with that.
The only thing you have to be concerned about is that down the road, if your insurance doesn't pay out on the accident, the other driver may sue you to recoup the damages.