The crop to the best of my knowledge is more in the chest, not the throat. It looks to me like this poor little one has been beat on by the bigger chicks. Look, I'm taking a stab here but stop the ACV, he doesn't need it. What he needs is some vitamins with probiotics to help build him up again. If he won't drink by himself, you may need to put drop after drop on his beak until he will do so on his own. There are some BYCers who do tube feeding but I have never done it and really would hesitate with one this small even if I had. I don't think you need the olive oil either. Using a syringe to feed gives me pause, if you don't get it in the right place he could aspirate and get pnuemonia. There may be things going on here not seen yet, and I hesitate to say anymore than I have since I don't know all of the symptoms or what the others died from. Need to know what you are feeding and anything else, no matter how far off the wall it may seem. Something is wrong here but what I don't know.