Hi Mic,
Your kitty needs to be checked out by a Vet. It is always good to have a new one taken for a Baseline cbc and health check.
A kitten who has been having diahreaha will be very dehydrated and needs to have IV fluids. Kittens often have worms and need to be dewormed --this can cause diahreaha, but there are other causes. Either way have him to the vets for a "new kit check-up" then you can go from there.
Keep us posted if you will.
CML