Hi there, so sorry about your chick. It's very difficult to determine a diagnosis as to what is causing all of these issues for your chick because the symptoms you are describing could be a number of different things. I will tell you that you're doing everything right, and unfortunately there's not much else you can do except for what you're already doing. If it's not eating or drinking at all the outcome does not look good for this little guy. But as someone else said, they need more water than food, so try to get a small dropper and get as much fluid into the chick as you can. Also,  it may be best to isolate the chick from the others until it's feeling better . Nature knows best. And if the other chicks are pecking at him, they know that something is wrong with him and it will only get worse and just stress him out even more. I know chicks thrive well together for heat, but with a heat lamp, he will be ok on his own. Also, as another person mentioned the straw bedding is not a good idea. I learned this the hard way when one of my chicks choked on the straw. So, if you're able to get shavings that will be much safer. Also, the feed. A chick starter feed, medicated or non medicated, whichever you prefer , would be much safer/healthier as well. Just because the other chicks are eating it ok does not mean he's handling it ya know? And a small bag of chick starter feed is only $10 and will last a long time. In the mean time, I'll do some more research and see if I can come up with some better info for you. Keep up the good work, prayers for your chick, best of luck to you hun!