I am sorry I don't have any pearls of wisdom for you, but I wanted you to know that you have cyber friends out here. I was so mad when I found out that when my H had his A and he took out three of my four children on two different occasions with OW and her two children. The children didn't know about the A but even so it still makes my blood boil so I know I would feel awful in your sitch. (((((((HUGS)))))))
Remember though that your son is young and adaptable and that to him everything is always 'normal' as long as those around him are happy and there is no tension. Also this new 'BF' might turn out to not be such a great guy with your W and it might just show her what she gave up when she left you. Personally, I think it takes an awful lot for one guy to take on another guy's child. Don't make this R bigger than it is in your head. You will always be your son's father and you will always have that connection with your W and her family.
My H's parent's split and both found new partners and yet they all still managed to get along well enough for the childrens sakes to always accomodate all sides at those annual special occasions; just give this time to settle and remember to breathe.
Saffie me 46 H 46 M in 1986 D20,D18,S16,D13 H's A 01/05 to 07/06 H recommitted to M 07/06 renewed vows 09/06 Going from strength to strength
