You can do it in 3 normal ways and 2 "Peculiar ways" (A.K.A.: My ways)
Normal Ways:
1 - Show him the first episode, if he likes the aspect of the adventure and him being rubber, he'l get attracted to it without being spoiled, which will make him like a lot the later arks. (Also, to help in the enjoyment, ask him NOT to watch anything on Youtube surrounding OP, so that he doesn't get spoiled, and the experience gets even better).
This plan should be used if:
-He doesn't mind immature humour.
-He likes adventure.
-He isn't a Naruto/Bleach/DB fan.
2 - Show him some awesome moments, showing him that this is not like DB, Naruto and Bleach, and its actually hard for Luffy to defeat someone, instead of "LOL, POWERUP AND VICTORY".
The best you could give him is Luffy VS Crocodile (Round 1) or Luffy VS AoKiji.
This plan should be used if:
-He likes Naruto, Bleach or DB.
-He thinks OP is kiddy.
-He saw 4Kids dub.
3 - Show him an AMV, normally it would be awesome to be one of an epic battle.
This to show him the epicness of the battles.
Should be used if:
1 - He thinks Luffy being a rubber kid is stupid and kiddy.
2 - He has an idea that the battles are awful.
3 - He loves DB, Bleach or Naruto.
And now, onto the peculiar ways, both of them work if the guy knows NOTHING about OP.
1 - Tell him the show sucks ass, but you should watch it because its funny to see how bad it is.
They'l most likely have low expectatives, and then go all "WOW, this is not bad after all, in fact, is fucking great".
Should be used if:
-They are not familiar with Shonen/Anime.
2 - Tell them the new successor to DB is One Piece, and it has like Goku except made of rubber.
This should be used if:
-They don't know nothing about OP but love DB/Naruto/Bleach.
The last 2 work ALWAYS as long as the requeriments are completed.