My 6 week old kitten is really poorly (I think)?

He is 6 weeks old and has totally gone off his milk and food! I have had him since he was 4 weeks and have been bottle feeding him substitute milk and he has got alot bigger but he has not had anything to eat or drink and I am really worried! I have recently put him on whiskers kitten meat and he also has royal canin dry food I have never owned a cat before and as he is my daughters cat I really want to male him better please help