My wife and I found out that she's pregnant. How do we go about telling her teenage son?

I'm 22, and my wife is 30. My wife has one son, and he's 13 (and is an only child). My wife and I married August, 2007, and we began having unprotected starting in November, 2007. Well, yesterday, she took a hpt and it came out positive. My question is, how do we go about telling him that he's going to be a big brother? He's a great stepson, and we have a great relationship...I just don't know how to tell him about this pregnancy. Thanks!