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.
Your question leaves me with a lot of questions. The long and short of it is, once you have appeared and been adjudicated and then not paid, you are considered a failure to pay at which point the case is assigned to collections and your license is on hold until the fines and fees are paid in full. That appears to be the place where you are at. Your license will not be released until everything is paid in full. You should be able to get a payment plan, but you may need to appear at...
Most likely you will not be able to get your license reinstated until the fines are paid in full. If you never appeared on the citation, you may be able to get a hearing and try to resolve the matter at trial, otherwise, if you have failed to pay, they want the money in full. Consider consulting with competent traffic ticket attorney. If not, get yourself to court and set up payments.