There are quite a few minor intestinal parasites that are fairly uncommon and that don't show up on regular fecal panels. Examples would be giardia (rarely seen on floats), coronavirus, cryptosporidium, etc. If your vet has not yet sent out a fecal culture (this is not normally run in-house), then he should.

Have they discussed Inflammatory Bowel Disease with you? You would need an intestinal biopsy to confirm that, meaning an abdominal surgery. It's not uncommon in cats...look it up.

And of course, good old food allergies, while uncommon in a young kitten, could be a possibility and would require an exclusion trial on something like Hill's z/d Ultra with no other food sources.

You should also consider a consult with an internal medicine specialist. Ask your vet for a referral.

If they are not pursuing this agressively, find someone that will. Not all vets are equal, and there are way more diagnostic options than just running a fecal float and shotgunning antibiotics in the hope that one will work.

Good luck.