It's not harmful but younger kittens may not respond to it. Some cats are 6 months or older before they respond to catnip. You can try it. . .I've had kittens about that age who liked catnip, but he might not be ready yet.
You can also try a few different styles of scratching posts. My cat loves a cardboard one that is shaped like a saucer. Some cats like vertical scratching, etc. try placing them near other places he's scratching.
My kitten is 3 months old and he's one crazy guy. A few weeks ago he didn't even look twice at the plant but today he pulled a leaf off and started to chew on it. He didn't eat any but I swear he's a little calmer. I was hoping it would go this way. Some cats get calm and other are a freak show! Good luck!!