OCGA 16-10-23: A person who falsely holds himself out as a peace officer or other public officer or employee with intent to mislead another into believing that he is actually such officer commits the offense of impersonating an officer, and, upon conviction thereof, shall be punished by a fine of not more than $1000 or by imprisonment for not less than one nor more than five years, or both.
According to the letter of Georgia law, under the described circumstances he could be guilty of that. I doubt that I'd arrest him, if he was just holding a sign, however: it's a felony, and I'd rather not stick someone with felony charges for just being a dumbass with some cardboard.