Invite him to play on the posts
You need to interest him in playing on the posts. And as I said, many posts seem to do the trick. I have never tried cardboard, but the sisal posts work the best for my cats. I don't like the carpet posts because I don't want my cats to enjoy scratching carpets!
If you are not going to committ to making it fun for the cat, then he won't give a hoot one way or the other.
It really does work, but you have to work at it too, not just buy stuff and expect him to "get it"...... you have to show him what they are for.....and you have to have more than one post ......
the end.