Is this just jealousy ?

Me and my wife have 5 children and our last one is on the way (due 30th september!) But our youngest is 9 and he is acting very strange he has been saying he didn't want mum to have another baby and he has been locking himself away in his room and kicking the door, he glares at myself and my wife and doesn't really talk to us. Our eldest (twins) said that he hasn't said anything to them either. We are very worried as Ollie has never acted like this before. Is this just jealousy and who can we control it? Thank you.