Oh I am sorry you are dealing with this. I have taken care of a kitten like this that was smaller than the rest of the litter. It appears to have fading kitten syndrome. Notice how the head is growing but the body is not.
There are some steps you can take, but it is a diligent task and you are likely to loose him anyway.
My guess, from experience, is that he is too weak to nurse, not even on a bottle. The kitten I nursed lasted 3 extra days, with a rigorous schedule, but I still lost him.
If you pinch the skin on the back of it's neck, does it retract very slowly? If it does not rebound quickly it means he is dehydrated. You can try to nurse him with a kitten bottle, but you will probably have better luck with an eyedropper, but I suspect that he is still too weak. If feeding the kitten this way, do not feed him while he is cold, and do not hold him upside down. Warm him with towels fresh out of the dryer, or a sock filled with rice warmed in the microwave, or even a covered heating pad. Feed him tummy side down and push the KMR/formula in slowly.
You can administer dextrose solution subcutaneously. This is best done by a veterinarian, but if you must and you trust yourself, it can be done at home. The shelter teaches us to do this for fading or orphan kittens. The supplies can be bought at a store that sells cattle supplies (or maybe even a feed mill). Use a very small needle, we use 3ml 21g1 needles and 50% dextrose solution. Suck up 3ml into the needle, pinch the skin on the back of their neck and insert needle through the skin. Push the plunger slowly. After they are hydrated you will notice a big difference in their actions! They might even seem like they are ok, but it only lasts a short time. While they are feeling well, try to get them to nurse off the mom, the eyedropper or whatever you can. And be diligent! It only takes 3 short hours for them to dehydrate again!
Lastly a kitten can be tube fed but you would definitely want a vet teach you how to do this and give you the supplies. Tube feeding can cause serious and fatal complications such as pneumonia and aspiration, and constipation/blockage of the intestines. This is a last resort, but often highly effective, method of trying to save a kitten.
Good luck with this little guy! I will be thinking about you!