I agree that a further discussion with counsel is in your son's best interest. Although I doubt you'd be sacrificing any rights or interfering with your son's legal defenses, it's best to have a thorough conversation with a lawyer before taking any affirmative steps.
Posting an answer to your question does not create an attorney / client relationship such that you can or should rely on the information provided herein to take action. Instead, it is intended to simply provide you with information. I am not your lawyer and cannot provide you with legal advice unless and until I am hired to do so.
Building a bon fire, so long as no property was damaged, is not a crime, unless there is some county ordinance against it. I would hire an attorney that does delinquency defense, and have them get with him/her.
As a former juvenile prosecutor in Palm Beach County, you should not sign anything without consulting with an attorney. In fact, you do not have to speak with the police if you do not want to do so and you should not speak with them without consulting an attorney. Should you decide to speak with the police, you should have your attorney present. It is impossible to give a more complete answer at this time since we do not know all of the facts related to the officer's request.