Will he really? What if he happens to agree with your viewpoint? What if you present him with a formal argument that is valid whether the premises are true or false? He can dispute the truth or falsity of the premises all he wants but it won't change the validity of the formal argument.
Well... Even if he actually agrees with your viewpoint - considering his sign, he'll probably play the devil's advocat and just act like he doesn't share your opinion
Go nuts