Open carry is not brandishing. There's no threat. It's a lawful activity. He didn't move the gun around when making a threat or gesture. All he did was have it over the shoulder, and likely didn't play ball with the cops who hate it when a "kid" knows the law better than they do.

It's worth noting that this isn't prosecuted by Oakland County. This is done by the City Attorney of Birmingham. Usually county prosecutes. This didn't happen.