I still think he's running scared! He's a tosspot for running straight into the arms of someone else, and she must be a super cow, but I still seriously think this is a fear reaction and he will come crawling back. From what you have said you have other kids to consider as well as the new baby and I personally recommend thinking about trying to work it out in time. He's made a huge mistake, but I think it is actually quite common for men to freak out and need to get away when the reality of pregnancy kicks in. Its just a knee jerk reaction and not necessarily a real end of relationship. But thats just my thought, and I would still make him suffer for ages before taking him back!!!
Do NOT get all needy and emotional when you see him! Be distant and cold and tell him that from here onwards your mum will be going to scans etc with you. After seeing the baby on a scan he will not want to miss the next one!