Need advise... and fast!

Kennedy
I have a son who will be 15 months old in a week, I'm currently 36 weeks pregnant. I don't know if he can tell that something is about to change or if it's just a phase but he has become quite mean. He constantly hits our 2 cats that we have and will chase them around the house with anything he can get his hands on. He smacks both his father and I even after we tell him to stop and that it's not nice to hit. I need advise on how to get him to stop before his sister gets here because I don't want him to hit her or be mean to her. Has anybody else been in this situation? If so what have you done to resolve it? I would really like some advise that doesn't involve spanking or yelling. As I'm afraid that he will think it's okay to continue hitting and start yelling when he doesn't get his way. My son is very smart and picks up on things very quickly. I'm desperate!! 😭