You don't ever bring a baby into a situation where the adults haven't fully stabilized. You shouldn't be having his babies until the two of you are comfortable in your poly. She shouldn't be having his babies *period* after a two month relationship. Babies need stability, and babies have to come before adult needs.
You have every right to point that out. You also have every right to tell him that if he wants babies, you want to give them to him and it is in everyone's best interest that if you really feel strongly about this, you tell him plainly that if he knocks her up, that will impact his relationship with you in a very negative way.
Your concerns are completely valid. Stick to your guns, don't be afraid to communicate with him openly about how you feel about it, and good luck!