As far as kids go it is tough. Last fall I went to lunch with a kid in my son's preschool class and her grandpa. I heard my son tell the little girl "My Daddy doesn't like my Mommy anymore" I was mortified. He also became very insecure and didn't want me to leave him at school because he thought I wasn't going to ever come back. Also, the few times H and I argued and yelled with eachother it caused 2 of our kids to constantly cry and beg us to stop and our oldest D's grades suffered terribly.
Sadly, this isn't just about you and your W. Your son didn't ask to be born, you both made the decision to have him. Now because the going gets tough your W cannot just decide that she doesn't want to do this anymore especially without trying to fix what she perceives is wrong in your M. These WAS's are so damn selfish and so are the OP when you come to think of it. Can you tell I'm mad?
Again, try to act as normal as you possibly can around your son, lay low and try not to put too much pressure on W as it only makes her mad.