Well, for one, there's no gaureentee he will EVER get a baby girl for starters, even if you do have his babies.
You guys aren't even serious enough for you to have him meet your kids, so he can't even possibly know what a house of children is like with you. Now, my husband and I were discussing these things on our second date, b/c we really wanted to know each other's life plans.
Just tell him up front, "You know, I really don't know if I want to have more children for a variety of reasons." Then tell him the reasons. Tell him you MAY be open to it later, but that you really can't make any promises.
Really, he waited this long to settle and start a family, so don't feel bad about that. Also, he deserves to know the truth. Having biological children could be very, very important to him and may even be a deal breaker. He deserves to know what he's walking into. He may be willing to give up that dream for you, or it may be something he could resent you for later in life if he does give it up for you. He may choose to break up... really you need to be honest with him and communicate openly and freely.