Only if he is wearing a kilt and playing the pipes.
I'm no lawyer, but it seems to me that he could fight that charge pretty easily. Here is the Webster's online dictionary definition for Riot...:
2 a : public violence, tumult, or disorder b : a violent public disorder; specifically : a tumultuous disturbance of the public peace by three or more persons assembled together and acting with a common intent
It takes 3 or more to make a riot. One might be guilty of disorderly conduct, but without accomplices one can not be partaking in a riot.
and if they argue that the law doesn't require 3 or more one can argue that the law is based upon a revised interpretation of the English language. Ignorance of the law may be no defense, but ignorance of the unique revision of the English language might.