My cat had babies and they are almost a month old. All of them are growin but one is about to die.its tiny?

I think that its just not getting any milk from its mother..all the other kittens have been hogging i guess..but this little kitten looks the same size it was 2 weeks ago..all the rest are big. The little one wont even move. It just lays there. like its dead but its not cause when the mother licks it it meows a little..somtimes he opens his mouth and his meow is silent..when we pick it up his head hangs like he has no control over it. We tried feeding it milky baby cereal out of a plastic bag.but he wont open his mouth. I think he is about to die..he wont drink his motherz milk either. What can i do to help him before he dies.