You can't change it, the kid is not going to evaporate. So you can either accept it or move on. I would move on because at the top of my list of qualifications for guys is NO PRIOR KIDS lol but it depends on what you want.
Well, there's nothing that can be done to change that. Are you okay with being with a guy who has a baby? If not, then don't say anything. If so, then say oh how cute, do you have a boy or a girl. And go from there.
What's wrong with him having a child? Now was he honest about the child upfront? If he was, then if you accept him then you'll need to accept his child. If he wasn't honest about the child upfront, then that shows his character as a man. What real man would want to be secretive about their seed! And if he wasn't upfront and honest about it, then you need to wonder what else will he hide. Nothing wrong with him being father, what should be important to you is what type of father is he. The way he takes care of and loves his child, this too shows his character! So ask yourself can you deal with a man who has a child?
Just think that you can be a Step Mother for the baby after your marriage or are you going to be a MOTHER for your baby and his baby... You don't think about how many children he has because He love his baby, you love him than don't worry you will love the his baby also. :-)