im so sorry your kitten has this .i dont know all that much about fcks , but i havnt heard good stuff about it. i know in many cases it reduces the cats life expectancy some kittens dont make it to 8 weeks old. i found the following infomation for you.
There are two major crunch points with FCK kittens - if the kittens survive onset of the condition they may survive, but breeders have seen kittens apparently doing well who die suddenly at 10 days, and others who thrive up to three weeks of age and then, similarly suddenly fade and die. If kittens survive beyond 3 weeks, then the likelihood is that they will not die of FCK, the ribcage may revert to a normal shape if the condition is not too pronounced, or may remain flat into adulthood with no apparent side-effects. A recovered FCK would have every expectation of a normal life, as long as the compression has not affected the development of the heart or lungs. If a kitten runs and plays as normal without getting out of breath or tiring quickly then it is probably OK - your vet may be able to assess the kitten more specifically. Some FCKs grow out of the condition and the ribcage goes to a normal shape in time.
Sadly, if your kitten survives the 10-day and 3-week times this is not a guarantee of future survival. Kittens who are particularly small have often got a compromised heart and they can die at 12-18 weeks, or even later. We all become very attached to these little surivors, and it can be heartbreaking to think you have saved them and then have them die on you just when you think you are out of the woods. There are no guarantees I'm afraid, but if the kitten is a normal size by 16 weeks and seems normal in every way, then it probably is OK.
Many people who experience isolated instances of FCKS, or have some kittens go flat in a litter but not others, notice that it seems to affect male kittens more often than females. Going by cases notified to me there certainly seem to be noticeably more male cases of FCKS than female, and several breeders have asked if it can be sex-linked, since only their males are affected. Possibly there is some relation between male kittens being bigger than females on the whole (breeders have noticed that often it is the kittens who start out being the heaviest and fattest who seem to get FCKS, not the skinny ones, however this is not regular enough to be reliable, and I don't have enough detail on all the cases notified to me to be able to give any statistical data on the possibility).
Since both sexes get FCKS the likelihood is that the preponderance of male kittens suffering from it is related to relative size and weight, and that it is not sex-linked. Possibly, if we ever get enough data about the problem, and can raise funds to do a veterinary study, it may be worth examining something like testosterone levels in affected and unaffected kittens.
hope this helps . i do hope your kitten will be ok sorry for the long post