gemini - first of all welcome to thecatsite. I am sorry that your first post and contact here is about such a sad situation. I do not have any answers for you as to what could be wrong with the little kitten - but you need to try whatever you can to capture this little fellow and get him to a vet ASAP. How old are the little ones? Is he old enough that something could have gotten lodged in his throat - or is he still nursing. How do the other kittens appear?
Others here will respond to your question with more answers than I have - but I do wish you the best of luck and would like to know how you make out - sending kitty prayers your was for a diagnosis and recovery for this poor little fellow!
Hissy told me that feral young kitties cannot resist a feather on a stick, etc ...and she was right. It has worked for me - I have captured 2 out of the 4 residing on my property...I got mine at the local $ store.