The fact they did not have their mothers comfort etc would not have helped. I don't like eggs being raised in that kind of way, it's unnatural - but, I'm not blaming you at all. My friends' school did that years ago, too.
I would have taken him to the vets. Vets are the only place you can get suitable medication and get him checked thoroughly. Rest in Peace, little one.
We will never find out what was wrong with him :( They suprisngly do very well without their mother, personally I think they do better. They learn independence quicker yet have us to help them :)
I hope Fluffy didn't die in pain :(
Sadly, it was probably just survival of the fittest. You said the other chicks were eventually bigger than Fluffy, and chickens tend to pick out the weaker/weakest one(s), and mess with them until they die. I'm not saying that's what happened, but possible. I have raised many chicks in an aquarium with a heat lamp (they would never have survived in the chicken yard with all the adult chickens, they would have been pecked to death), and they turned out great. It's surprising how the death of such a small creature---sometimes meaningless to others---can impact us so hugely. I am so sorry you had to go through something like that, but it just shows you that no life is insignificant. Good luck with your other chick.