I don't think there will be any quick fix, as these things take time.
Just realise the most important person here is your little boy (which I'm sure you already know :).
Don't spend time blaming her, although morally she may be questionable, your partner is the one who responsible for hurting you. I mainly say this as blame and hatred can eat you up and take up a lot of unnecessary time and energy when you have much better things to concentrate on (not trying to excuse anyone, just trying to help you).
Concentrate on making yourself and your son happy and think rationally about the answers you want from your fiance. I realise this isn't easy to do at this time, so maybe having some space from him will help you clarify things. If you have been together a long time, a month or so apart won't be the end of things. If he has decided to move on, then so be it. I do however think you deserve an explanation and that he is being weak, so maybe after some time to think about your relationship from an objective point of view you can ask him the questions you want, and deserve, answers to.
This is all really hard to do when you are hurting. Have you got any friends you can go to to have a good cry and a rant? They might help you see things in a new light.
Hope things get better soon, just think of your and your son's welfare first, it will give you something to focus on.