Discharging a firearm without the presence of doughnuts?
Seriously though...if he shot it from a flare pistol, and someone reported seeing a "gun", that's all it would take. A firearms violation versus a fireworks launch. Flareguns are often tolerated, but technically fall under firearms regulations.
In some places, especially Miami and Detroit, there are major problems with folks shooting real guns on the 4th and a good real zero tolerance policy. Dummies don't understand, the bullets always COME DOWN and hit something, often someone. So when the cops hear "gun"...can't blame 'em.