Maybe that is why the girlfriend ended the relationship, maybe she was getting the feeling it was a bit of a menage a trois. She may have always felt like second best and that his mind was on you?
Maybe this relationship can be repaired; and that is not to say that you will never fight, never disagree, never hurt each other with mean words when you are cross again, but it DOES mean that you have both made a choice to stick it out thick and thin. And you are right, you have to make a pact that that is what will happen as your son will be screwed if you split again, it would be heartbreaking.
I hope that you do work things out. I think it could "restore the years the Locusts have eaten" and just hink of doing the wedding again....yum yum. Then again, I am a sucker for a fairy tale....
The best of Luck to you and he.