I know this isn't the popular opinion here, but some cats will do the opposite if you put their paws on the post and move them in a scratching motion. I prefer giving him a stern "no" and then showing him with my own hands how to scratch the post. When I see him stratching on the proper stuff I get melodramatic praising him for his good deed. They love to please. My cat has vertical, floor and both the rope and carpet. He tends to use the floor one the most, but likes the others also. Giving them a choice helps. What to scratch is learned in the first 4-5 weeks of life so sometimes it is difficult to break bad habits.
Cat nip is a wonderful draw when he gets older, but when he is under 6mths it may not effect him much!