I tell him that. He gets in debates at school. And since he gets frustrated and can’t make his point they all pile on...they do it just to have fun watching him get spun up. I know their type...I’m one of them. I am always getting people spun up just to see their reaction. So I know exactly what my son needs to do to diffuse it and not feed them, but he wants to be like me in debates so it’s easier said then done for him.
Since you can't possibly be there to provide coaching, perhaps you can give him a few stock "exit" phrases to use. If his friends say, "we think Obama is a saint and his policies rock!" Teach your son to say something like, "I guess we'll have to agree to disagree on that." My original advice still stands...if he can't yet make a succinct and cogent argument, don't be goaded into a no-win situation.