Two observations: (1) You have legal counsel representing your son, he/she is the person to whom your questions/concerns should be addressed. (2) As far as I can tell, the matter is still pending, so I'm confused about your criticism of the process before it has concluded. Don't underestimate the significance of your son's actions. I'm glad to hear that he understands its significance. When emergency services respond to a false alarm they are obviously unable to respond to a real emergency that happens close in time to the false alarm. Is it possible that serious harm could flow from such a distraction of emergency services? I think the question answers itself, and certainly if there were no consequence for such action it likely would become very popular thereby guaranteeing that eventually someone would suffer great harm as a result. I hope it works out satisfactorily for you both.
First, you have a lawyer and should be talking to them. Second, attitude of the parent can make a huge difference in how the state proceeds, whether its as a child in need of supervision or delinquent conduct, because there is a big difference on formality of the punishment and lasting consequences. I would suggest a more appropriate attitude towards the process. Again you should speak to your lawyer about these issues. Third, I can only imagine what the cost of summoning a fire department is and in these days of tight budgets you may be about to find out how expensive. Again, current counsel can help steer you through these issues.