Sounds like he did report you to the police and they will be charging you. Dont send any letter or speak to anyone without the presence of an attorney. Hire the best criminal defense attorney you can afford as soon as possible to mitigate the consequences.
If this is the only conviction you have, then you could have the record expunged. Contact a criminal defense attorney for assistance with the process. Most offer free initial consultations. Avvo is a great place to start your search for an attorney.
No way to keep this off your record besides fighting it at trial. Contact a criminal defense attorney to determine if the police used the proper procedure for the stop and search and whether you have a defense.
Fees vary among attorneys, you need to contact several local criminal defense attorneys for a private consultations to review all the facts and provide you with a price. Most attorneys offer free initial consultations. Due to your sons age and lack of prior convictions he qualifies for HYTA, a program that can keep this off of his public record. I wish you and your son the best of luck.