I'm jealous of my own boyfriend being a father figure

Soo long story short my boyfriend is a father figure to a child that isnt his. When he was dating the mother of the child he thought it was but later he got a test done that told him that he isn't the father. Like I'm upset, I honestly don't even know why I'm upset. I guess it's the fact he is already a father to a child that isn't OURS. Idk I told him I thought I was weird. He even has this child's name tattooed on his chest. Idk I feel like I'm overreacting but when ever he tells me he is about to take the child somewhere I just feel some type of way. The child even calls him "daddy" like. It just bothers me. I know I know it's wonderful that a man would father a child that isn't his. I kinda wish i would have asked him more about him being a father figure before getting into the relationship. I love him to death but deep down inside i cant stand that he is fathering a child that isn't his.