My cat isn't eating, drinking and moving much!?

My cat just lays down wherever he was placed. His siblings were still normally playing and eating. (There're 4 of them and they're 10 weeks old and can already eat). He doesnt eat or drink or even move so I had to feed him water with a dropper thingy manually. He was already like that even before one of his brother died (there's 3 of them now). He only moves normally when he see his mother (he wants to feed on her), but the mother doesn't want him to drink from her and runs away. I dont know why she runs away. He has also vomitted twice. It's either he's sick or is it deppression?