Can you get some colostrum? I suspect the litter had not nursed, or nursed enough, or the mama was very very debilitated. Get some colostrum into this kitten if you can. Small, very frequent meals could help. I'm at a loss where you might be located -- do you have access, maybe, to homeopathic remedies? Carbo Veg would be one to try for him. (This is a HORRIBLE example of homeopathic prescribing by a non-certified professional but it's an emergency so I'm throwing caution to the winds!). Remedies can be found (in the US, at least) in health food stores, at new age sort of businesses sometimes, or from a homeopath.
Jocullen, if you are not completely devastated by this -- Hissy's right, tiny kittens are both resilient and fragile at the same time -- you should make a point of forming a relationship with a good vet who is committed to helping with rescue work at least through phone consults. And get a baby scale or postage scale. Weighing them is one of the simplest ways to keep tabs on how tiny kittens are doing, or when they are in trouble.
You aren't tube-feeding, are you? If so you need to be very sure you're not directing formula down the kitten's WINDPIPE rather than into its stomach. If the kitten will nurse on a nipple or even a syringe that's the way I prefer to go. Could you keep the kitten inside your shirt or next to your body for warmth? I'm a firm believer in touch helping sick little ones to get better, too. I think part of it is sensing the beating heart, it makes them relax some.
Best, best of luck to the kitten, and you