Spoonman...
As several folks have already recommended - get an attorney. As the eldest son of an attorney, many, many times have I witnessed the futility of "remedial" law when a little up-front, "preventative" law would have resolved the entire matter. It's money well spent, considering the alternative. I'm sure the lawyers in our midst would heartily agree.
From a strictly amateur detective perspective, I would ask - can the DNR officer prove that you and/or your son moved the target? And, can he prove that any or all of the empty shell casings were left by you and/or your son? In other words, can he produce any evidence (fingerprints, etc.) to support his accusations? Was he an eye witness to your transgressions? If so, where was he situated? Given what you witnessed at the range, could he have observed any of this without you or your son seeing him? Any reason he couldn't/wouldn't have cautioned you at the time, assuming he actually observed you?
If you and your son haven't already, make LOTS of notes - when, where, who, why, what, times, dates, circumstances, etc., from start to finish. If you actually wind up in a courtroom proceeding, your memory won't count for crap. Odds are, the DNR officer has made few, if any, notes and you (or preferably your attorney) and your notes can make him look pretty foolish in front of the judge.