Your circumstance is, indeed, a complicated one--far too complicated to address in this format. The DA is not on your side. He or she represents the State of North Carolina and does not get involved in private matters such as this one. Other factors may come into play, such as your son's age and the extent of property or personal injury damages.
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Generally, a conviction of felony death by vehicle results in a permanent suspension. You may be able to obtain a conditional privilege after a period of time. You do need to consult with a lawyer about his matter.
While a speedy trial motion certainly should be considered, it is also necessary to lay the proper ground-work for that motion. Speak with you lawyer about requesting that the case be calendared for trial.
Additionally, it may be that the payday loan company has passed the statute of limitations for such actions.
Generally, you will need to contact a lawyer in the state where you wish to have the record expunged. Most likely you will not have to appear. Costs vary, of course, from state to state and lawyer to lawyer but it will be worth it to rid your record of this minor matter.