Wowza, what a thread...am going to try to to ignore some of the side-debates that have arisen just focus on the OP here but there are some very provocative comments...
OP, I don't think you are BU to have these feelings. It sounds like you have been through an exceptionally hard time...god, I can't imagine having to deal with a child having such an awful illness, and to be coping with that on your own AND the shock of your ex leaving...really, there are no words for how horrible that must be. Your ex sounds...well I'll settle for extremely twattish, but I want to say a lot more. It seems that you are well shot of him.
I do think it would BU to direct fury at your ex's new wife instead of the ex himself. From what you have said, she seems relatively innocuous. He should have told you he was married - not her. TBH if she is staying in the car she's already showing some level of sensitivty towards you, at least.
As a stepmother yourself, you obviously know the importance that stepmothers inevitably play in childrens' lives, whether you ask for it or not. It must be hard to do it with someone as twattish as your ex, but I think for your DS's sake you have to try to foster good relations with them both at some point...it'll be so much better for him in the long run. But anyway, from what you've said you already know all this on some level. Honestly...it sucks that you have to play nice with a man who has treated you so insensitively, especially as you've already been through so much...but unfortunately you have to do it anway IMO. Life is just not fair sometimes....