If he admitting to firing into the air, perhaps that would constitute negligence. It doesn't state that he did, just speculation. All I'm saying is that it's possible he said something else to incriminate himself. On the other hand, if they cannot prove he fired in an unsafe direction that would likely cause harm, I think he could beat the charges.
Years ago, I was shooting on a friend's property with permission and a local LEO showed up to investigate a report of gunshots. He basically asked which direction we were shooting and after we replied, he looked around and said "OK, have a nice day, be safe. Keep it in that direction." There was no ordinance in that town regarding discharge though.