Do you have any more information you can share? The average height for a cat is 8-15 inches so your cat is only a little on the short size. Is he healthy otherwise? Eats well? Good exercise? Your vet can tell you for sure but if everything else is fine then I wouldn't worry about his height.

He's outside during the day and he runs around with my young puppies so that is good exercise, and he does eat well.
And besides the things already stated, there isnt much more to say about him. Although its weird because his sister (we got 2 baby kittens at the same time from the same mother) is the same size as him aswell.
I've heard of people's cats staying pretty small, but it just seems a little odd.
I added a photo of the little guy when he was younger, he is merely the same size even now.

Hi, Don't worry about his height, He will get his natural height.