Husband is soooo mad

So my boyfriend of 7 years just found out his cousin and gf of 6 months are getting married AND expecting their first child in July. He just got divorced a year ago. So when he heard this news, he was so pissed off. We are not married and don't want to be. We do have one child together and he does not want anymore.
He's not really close with his cousin but I don't understand why he's so mad over this news. I'm bored and he's being crabby in the other room so wanted to ask this. I asked him WHY and says he's not mad