Well., first of all... thanks for taking care of a sick... no matter it's a cat or a human, you deserve kudo's for that and you will surely be rewarded for your good karma.
as for cats.. I have so many... nearly 20 of them

... so trust me when I say this " that kitten of yours seems to be dying one"
What you have to understand is that, sometime kittens before they were born fight for food and nutrition in the womb... and this kitten is the one who lost out on the battle with his/her siblings.
Usually on a pack of kittens, the weakest die within a day or within the week.
but if you are really keen on keeping this kitten alive.. do the following
1 .Put up a high density light or a keep him under a tungsten bulb to keep it warm..
2. See for about 2 -3 hours and if it doesn't sort out the problem, take the kitten to the vet and ask him to give the following
dexamethasone - it's a Steroid shot that will jump start his system. The vet will know the correct dosage...
if that fails with in the next 3-4 hours to revive him out of his erratic breathing .. ask the Vet to give him a "Adrenaline" shot.
if that fails.. well.. nothing else you can do about it.
Don't try to give him "cow milk" which is Toxic to kittens that small.. try always to keep the kitten near the mother and guide him to the mother cat's nipple and help it to suckle.
Hope it help and fingers cross mate..
all the best