So sorry. terraplnr. I can't believe the schmuck is walking out just when you and your child need him the most. That said, a few suggestions:
See a lawyer immediately about child support. You won't have time after the baby comes.
Line up as much help with the baby as you can. Your son will need extra attention from you with the new addition to the family.
I'm not sure how old your son is and what activities he participates in, or could when he gets older. Whether it's playing sports or Scouts (or a church-based alternative), be frank about being a single mom and your son having little or limited time with his dad. Plenty of people will step up to help. With my son, a good friend's family included him on day fishing trips. His den leader and family had us over for dinner so my son could work on his Pinewood Derby car with the leader and his son. The dad next door taught my son to ride a bike. My brother invited us to join his family on inexpensive trips to scallop in the Gulf, and taught my son to shoot and hunt. My mom taught him to play poker. That's just the short list.
I don't know how you stop thinking about the other person. My situation was very different, but a therapist just told me to keep telling myself "He's insane, he's insane" and that worked quite well for me. I quit trying to analyze what happened. Because you'll never figure out the real "why".
My sympathy as well. I'll echo what ZG said, with the emphasis about trying to figure out why.