You don’t bring it up. Neither one of you is ready to have a baby. Your focus should be on each other and making your relationship as deep and solid as you can. Babies need to have parents who are fully committed to making a life together. Nowhere in your letter do you mention such a commitment or marriage.
At 22, your biological clock for bearing children is certainly not ticking. I have a guess that the talk about babies is more about wanting to pull your boyfriend closer to you. He does have a big commitment elsewhere. His other child will always be his first one. He will always have responsibilities toward that child. You can’t compete with that and you shouldn’t.
At some point, you will also be one of the adults in that child’s life. She or he will be spending time with the two of you and there will be times when you will effectively be a co-parent. It’s important for you to embrace that as an opportunity to love and make a difference in another young person’s life. Your relationship with the child and with your boyfriend, and your success as a parent to your own biological child will be affected by how you manage it.
When your boyfriend says he already has one, it is probably his way of telling you that he isn’t ready to add emotional and financial responsibility for another child to his life at this point. You should be in full support of his involvement and financial support of the child he already has. How he manages those obligations will be reflected in how he handles parenting a child you eventually have together.
Unlike your boyfriend, I don’t have trouble telling you that a pregnancy at this point is a very big problem. It’s unfair to your boyfriend, to your future child, and to yourself. You have more growing to do before you can be the kind of mother every child deserves.
I wish you well.