need help!

Okay so I've been with my boyfriend for 2 years, today he told me that one of his exs called to tell him that she had his kid 3 years ago, and that the kid is going to be turning 4 soon. Idk what to do TBH, I'm going to support him and what not, but idk about how I feel about him having a kid that's not mine. I'm sorry for saying it like that, but that's how I feel