I'd like to ask you, was your kitten ever tested for FeLV/FIV? If so, how long ago and what were the results?
In this situation my vet would recommend an immediate retest, regardless of previous results. (This is normal procedure, and done even on older, strictly indoor cats when they become seriously ill. Checking for an underlying infectious disease is very important in such cases.) Please talk to your vet about this as soon as possible.
Also, when was the last time you took a stool sample to the vet to be checked for worms and parasites? If you didn't do this very recently (just a few days or maybe a couple of weeks ago), please do it early next week. Worms and parasites can do terrible things, especially in young kittens.)
I'm wondering, has your vet done a complete chemistry profile and CBC, or just some partial testing? If complete bloodwork hasn't been done, that would be another thing to discuss with your vet and go ahead with immediately.
You should do all these things and, if by the time everything is done and your vet still doesn't have a diagnosis, please ask your vet to refer you to a specialist without delay.