What can he do to get a permit for the sign? What are the applicable regs.? The city cannot constitutionally regulate the content of speech, only the time, place and manner, and that reasonably. And if the city's pattern of enforcement of its regulations is biased, that might be a constitutional claim as well.
LOL! You've hit upon the irony of the situation. They're having a RAT convention and there will be plenty of supportive signs both inside and outside the building. Yet across the street is a counter-political sign, and it is prohibited.