Personally I don't believe that if someone cheats once they will do it again. If for example, you were having relationship difficulties or he was having problems in some other area of his life, then I can rationalise that a person may temporarily fall into the arms of someone else as the escapist method of dealing with the reality of their life at the time BUT he hasn't done that. He has maintained a relationship with someone for over two years and hasn't just had one reckless moment. It is possible to get over cheating but only if both parties want to and there is transparent disclosure from both sides about what caused the infidelity in the first place. Also, while I don't think people should necessarily walk away from difficult situations, this is something that will affect your life forever because a child is now involved.
You have to ask, would have have even told you about this if he had 'got away with it' and had not conceived a baby with the OW or would he have maintained the secret and, if effect, you would have continued living a lie?
I really think, in this case, you need to put your concerns about your DC to one side and think about what you want, how you imagine your future being and whether you can factor a sibling of your DC being involved in your life forever? Perhaps the best thing to do would be to say everything you have to say on the matter to him, find out what you need to know (his explaination of how and why he started having an affair) and then give yourself some time to think about what you really, really want. Of course, he may decide to invest in the relationship with the other woman if you are not giving him the attention he desires but that would be his choice and would speak volumes. You also may decide that the reality of not being together is not as bad as you feared and may want the relationship to end.
Personally I think your desire to save your marriage is a knee jerk reaction to finding yourself in a situation you did not imagine you would be in and you are trying to hold on to your life as you know it but it will never be the same again. Are you prepared to shape your future around the constant reminder of his betrayal?