We can all offer advice but none of it is a substitute for a vet who does a thorough work up and who is able to work closely with you and your cat.
Negative tests for giardia and coccidia are not a guarantee that the cat is parasite-free. Personally I would also be treating him with Albon (for coccidia) simply because if you play the odds, his problem is either related to one of the above mentioned diarrheas, and you've only been treating him for one of them (if I've read your posts correctly). A common schedule of coccidia treatment is one week on albon, one week off, one week on, but vets use different schedules. Bare in mind that *if* it is coccidia, and it is not treated, it could kill your kitten. I have seen this happen. I'm not trying to scare you, but I want you to be aware of the issues.
Other possible causes could be some form of colitis, or IBD (Inflammatory bowel disease) but I honestly don't know if your kitten is old enough to possibly have either of those.
Since this is a shelter cat, the chances of it being a parasite are high. From what I've read, you have never treated the kitten for coccidia. Is this true? In my opinion, a shelter cat with undiagnosed diarrhea that is not responding to other treatments should be on Albon for coccidia if for no other reason than to rule coccidia out as a cause. I don't want to criticize your vet, but if you haven't treated with Albon yet, and there is no diagnosis, then there is no justification for your cat not getting Albon. The odds of it being coccidia are too high to ignore, fecal results not withstanding (and not hugely relevant). This isn't rocket science. Shelter kittens with diarrhea are 95% (99%?) likely to have either worms, coccidia or giardia, and if you have only treated for giardia, then you're not treating for the other, overwhelmingly likely cause (coccidoa). I'm assuming he was de-wormed, right??
There is no downside to treating the cat for coccidia. Albon is completely safe, not very expensive, easy to administer, and doesn't taste bad.
If you have treated with Albon (yellow liquid by the way) then its a different ball game, but again i'm assuming you haven't based on your previous posts.