Just tell him it's alot like acting, only you get to kill things, you don't get paid (not a huge leap for actors! ), and you have fun, cause the director usually doesn't yell at you.
Remember when you used to play make believe as a kid, you might have played cops and robbers, or cowboys and indians.
Do you remember a time when you "shot" the bad guy and they claimed you missed? Arguments would ensue.
Roleplaying games are like make believe but with rules to handle these kinds of disagreements.
But I have always found the best way is to let them watch a game, perhaps have a couple of DM-characters ready to give them a little role here and now if they're up for it.
It's:
*Two(2) heaping tablespoons of Lord of the Rings
*1/4 cup Who's Line is it Anyway (improvisation)
*1/3 cup social game
*....I'm not a very good cook so I'll stop
Hopefully if you bake that for a while, you get a D&D pie. Mmmmm....koboldy...
Don't use the "lets pretend" example. It only makes things sound worse
Trevalon had a good idea though I would say:
One part your favorite movie
One part "Whose line is it anyway" or "Theatre Sports" (if they have the former program
One part hanging out with your friends
One part living out you fantasies
I always refer it as acting, and use literature and movies as additional tools when needed. And of course there's always the "it's a great way to spend time with friends" explanation.