I agree with the other attorney's answer, except for the part about the Judge not knowing about your other case when you go in for the speeding ticket. If the DA has filed, some courts are so connected that it will show as an open case. The Court will most likely just remind you to take care of it.
The letter is hearsay and won't be considered. If you want to testify at sentencing that might make a difference but not likely. YOu say he has turned his life around and it sounds like he has done a lot in that regard, but I have to ask : What part of NOT own or POSSESS A FIREARM did he not understand. Naive is an Excuse.
Has your son pled to this two year old allegation? The answer depends on the status of the case. It is illegal in California to have an open container in a vehcile regardless if the car is being driven or not. We would need more information in order to answer your question. I suggest you seek a consultation with competent legal counsel.
You need to contact a traffic lawyer in your area. We are able to walk you through most of the steps and some of which you may want not to handle yourself. Contact a lawyer in your area promptly. If you have the fix it's fixed, you can get those dismissed. If you don't have the vehicles, you need to get proof of otherwise a game plan to address all your citations at once.