Open carry of the rifle by someone under 18 is only a class a misdemeanor in Wisconsin. Even if he is found guilty of that, he does not lose the right to claim self defense under the doctrine of necessity. The only thing he could lose, and I am not sure about Wisconsin law, is stand your ground. Since the guy had a molotov cocktail, there was no safe avenue of retreat. Before the conflict with the other rioters, he was retreating until he fell down and at that time could not safely retreat.