He needs to be dewormed. You can typically buy dewormer off the internet, but if you dont know what you need, check with your vet first.
Also, keeping a cat outside will increase its chances of getting worms. And worms can be very harmful. (Not saying you let your outside, but just in case)
He probably has worms and needs to be seen by a vet so they can tell you what to do. Get him to a vet ASAP!
PS Try to avoid touching the worms and the kittie's "dirt" with bare skin. Use gloves and wear a mask when cleaning his box, sometimes people can get stuff (parasites, diseases, etc...) from their pets.
Take your kitten to the vet to get dewormed.. Usually kittens are born with round worms and hook worms and round worms.. So, they will need antibotics and shots to keep the worms from spreading/breeding and eventually killing your kitty...
This called a tap worm. My kitty had them even after he was wormed. Cats can get them from eating old food, not changing the litter box frequently or even being outside. Go the Vet and ask for a de-wormer. Very cheap and very much worth it!