How do I coup to my son about him being a brother?

Hi mom's I have a 3 year old son who is just amazing on listening, doing his chores, cleaning and helping me out with everything! Even when he plays, after play time he cleans up! Will my problem is my husband and I tell him his gonna be a big brother to a sister and I'm 6 months pregnant, but he completely ignores us about. If we bring up the word baby that doesn't envolve him he completely shuts us out. Why? And I don't even want this baby to change him, I want him to be still be himself, we haven't even bought baby stuff, what do I do?? He will be almost 4 when she is born. Did anybody else have problems like this with their first kid?? Please and on top of that I'm very emotional and it breaks my heart my son being like that.