Jealousy

So Im currently 29weeks along with a boy. I have a 4 year old cousin that I am around a lot. When we all told him ibwas having a baby boy, he started acting out really bad. Then, he started actually punching me in the stomach and whenever I lay on the couch he'll jump on my stomach or try to bounce on it! He even tried stabbing me with a key a couple weeks ago!! I seem to be the only one yelling at him and trying to correct his behavior. Im terrified to bring my newborn around him when I have him. My uncle makes jokes about him throwing my baby which i happen to not find funny AT ALL. He is the youngest boy in the family and im thinking thats why he acts like this, however, its no excuse and it really is not okay. Its scary. has anyone else had a little one THIS jealous and how did it go when they met the baby?