Help please don’t be mean please

I’ve gotten back together with an ex boyfriend and we’re better for it. We’ve talked about the future and we do want children that hasn’t changed for us. I found out that I’m pregnant with our child and I don’t know how to tell. The kicker is that he’s having trouble from an ex of his that is claiming her daughter is his (baby looks very very different from him. He’s white as can be along with the new mommy but the baby is a very darker color than both of them) I don’t want to add to his stress dealing with that too but I want to tell him I just don’t know how to.