My younger Cat attacks my older cat how can i stop this?

Last summer we got a 2 month old kitten so our 9 month old could have some company. The first day or two the bigger cat was a bit jealous but he got use to it.They were fine until Lilo the baby cat started to get bigger and now he is 1 pound heavier then Diego. They go at it every night and I know they are not playing because the sounds that they make, the funny thing is, my older cat will make all kinds of noise but he will not defend him self. We even neutered the younger but it did not help at all.