It's two days old,I don't even think he has opened his eyes yet in this photo. When a mother cat(Or even a human grabbing a cat of any age's neck),Like stated below it deactivates them. It isn't hurt. The mother is either carrying it for convenience,or it wandered off. I used to do this to my kittens all the time. Heck. I still do and they are 6 months old! My first was feral,So it taught him discipline/Obediance. The other liked to climb places she wasn't supposed to. That was discipline as well.
When a mother feline grabs their young by the neck, this sort of "Deactivates" them. Rendering them in a state of paralysis. Useful and safe for the kitten to not move while the mother is transporting. This can also be done to adult cats as well. During veterinary procedures a paper clamp is put on the back of their neck to keep the cat from moving while it receives a shot or something. It does not hurt the cat whatsoever and it is a very amazing phenomenon.
I used to do this to my cats when they wouldnt listen to me very well and wouldnt listen. Hold it for a moment then say no then let go, well soon my cat like jumped like it hurt her when i did it, then she died!