Please be careful. I don't think a new baby is something that you need right now. Perhaps he is sincere about getting back together, but I don't think these are the right circumstances to have a new baby. Reconcile first and then take it from there. I'm sure you have had a very difficult time trying to raise your baby by yourself, multipy that times two with a new baby.
Tell him that although you would love to have another baby you don't think you are ready either physically or emotionally. Tell him you want your baby to have a chance to be spoiled by both parents and be a baby a couple more years. He needs to bond with this baby. Let him know you want to work on the marriage first.
Life's challenges are not supposed to paralyze you, they're supposed to help you discover who you are. -- Bernice Johnson Reagon