Fever coat? I haven't ever heard that term, how interesting. That has something to do with the temperature of the cat changing the color of the fur as it grows in?
And once the cat (or, kitten, in this case) is in good health, as he sheds the fever coat and the new fur comes in, it will be a normal black color?
PS to the OP, is that a flea collar you have on him? I strongly advise you to take it off, if it is, those flea collars are known to be toxic. When he's a little bigger your vet can give you a kitten dose of Revolution to safely treat him for fleas.