It is protected speech, even if it reveals the guy to be stupid. The wonderful thing about free speech (and protecting the speech of those who choose to speak stupidly) is that there's a free market that can force him to pay for his choice of words -- if it believes them to be problematic. He can post inflammatory signs outside of his restaurant if he chooses, but those who pass by it can choose not to patronize the restaurant because he said something offensive. It's a far better check on speech than anything the government might do.