Puppy and chickens?? Help!!?

I just got a 3 mth old australian shepherd.I also have baby chickens outside that he likes to chase and nip at. i have been hitting him on the nose and telling him no, but he just doesnt understand what im asking him. He continues to chase them. And i cannot have him kill them, because then my dad will make me get rid of the dog. How can i teach him not to harm the chickens? I really need to correct him fast before i cant correct it. And it becomes a problem. if you have more than one method, thatll be great.