Welcome to TCS. There are a lot of knowledgeable, friendly people here. Unfortunately the mortality rate of kittens is quite high and sometimes, despite all our best efforts, there is just nothing we can do to save them.
Here are a few things you can try:
It may be that the little guy is having trouble latching on, try giving him some private time with mom and see if he'll latch on preferably on a hind teat. You can try to entice him by placing a little corn syrup on mom's nipple.
If that fails, you can try feeding him yourself with some KMR (kitten replacement milk) I imagine he will have a difficult time with a bottle, so I recommend a bulb syringe/medicine dropper. Never feed a cold kitten so be sure that the kitten is warm prior to feeding it. Always hold a kitten tummy down when feeding it - as if it were nursing. Holding a kitten on it's back could cause it to aspirate the milk and drown.
If the kitten is looking dehydrated, you can administer glucose intramuscularly. You would need a glucose solution and very small gauge needles. The shot goes into the skin on the back of their neck, if you are comfortable with this, PM me and I can explain the process in detail more.
To tell if the kitten is dehydrated, pinch the skin on the back of his neck. If it rebounds quickly, he is not dehydrated, but if it takes awhile to spring back, he is.
Lastly he could be tube fed, but I recommend a vet show you how this is done. There are some risks associated with this treatment, but if the benefits call for it, it can be life-saving.
I hope your little guy turns the corner.