Sorry, IHH, I didn't mean to "comment and run" with my last post but I was headed out the door and have been most of today.
My particular thoughts were that from a legal standpoint it might be good information for you to know in any sort of Separation Agreement or divorce proceedings as to if he is going to be legally liable for this child by OW.
I agree with Allen that with H's other behavior, the pregnancy issue doesn't change your actions - as to getting out of the current living situation. H has done enough to warrant that but the pregnancy thing just adds fuel to the fire.
However, I would think it would be definitely good to know if he is fathering another child, personally, for several reasons. The main one would be to protect your/your son's interests. The 2nd would be knowing what you're up against.
Others have already suggested ways in which you can try to get proof. Do H and OW go anywhere in the car together or does he talk to her on his cell? I know it was suggested to me that I get a voice-activated recorder and put in the car to see if I could catch H in a conversation. You could possibly find out that way.
I do agree with Allen that whether or not OW is pregnant shouldn't affect what you do in terms of the M or living arrangements.