Kitten w/ runny stools?

A negative fecal exam does not necessarily mean there are no parasites present. Various diarrhea-causing protozoa, for example, coccidia, can be missed on such an exam. You might want to ask a vet about trying a round of anti-protozoal medication (different than a de-wormer or antibiotic). There are other possibilities as well, with the list ranging from most-likely to least-likely causes. And by the way, the answer might be as simple as switching to a regular old dry-kibble kitten food. Be mindful that the kitten doesn't become dehydrated from the diarrhea (this can sneak up on him fast) while you are trying to sort out the cause. Best of luck to you. Hope the little guy gets over it soon.