Show me an intermediate guitar player who can play the entire song down stroked like James plays it. Beginners can’t even get past the intro riff because the amount of required down stroking kills their arm. If a song has so much down stroking in it that even intermediate players still can’t play it properly, then that in itself is metal.

Do the same to him. Find whatever song he plays a lot, look it up on YouTube until you find someone else playing it differently than him then tell him he doesn’t know how to play and shut off his amp. Might cause a freak out the first time until he realizes you’re just showing him his hypocrisy.