Hello Crimson, as you see in what you wrote, you want to save your M , your W and your S, people does things that sometimes in the moment we dont understand and its fine, time will give you responses.
I dont have kids so I am not really in the possition to know how you feel, but I have compassion for you and you as a parent need to underestand something, as much as it hurts and you dont like it, your kid has to go trough this by himself, of course its not an ideal situation but its the actual situation and you have to accept it.
Sometimes we get to involved in the pain or suffering of others, stressing ourselves to find a cure for their pain...well thats not gonna happen in a emotional level, he has to see his world around as something that leads him to change, you have to set the example because in the early years is when kids keep chhosing role models you understand?
Instead of suffering the situation, something that affects your son, validate his feelings and accept them, he has to learn that his choices are valid as well, if he doesn't want to spend time with his mom at this time its fine, its the way he is dealing with this, you cant push him to see this in a different way because you are not helping him like that.
Set yourself as a leader, a pleasure father to be around, one that builds his confidence, if he doesn't want to go to mom, ask him, what would you like to do instead? What is that you would like to do? And do it with him, or take the lead, show him that life has great things.

Now the attitude of your son is basically what he sees in you, you are angry and upset with your W, your son its afraid and new in life so who could he copy right now? Yes, you, thats why he is upset with mom, show him how not to be upset at life and how to have great times with you, thats what your son its choosing now, you...and thats his choice, even if your W doesn't like it.
What happens many times is that we enter a circle of guilty, oh I feel bad I did mistakes in my marriage and that because of that my son feels sad, and the circle keeps spinning, like a mousse in a cage, set yourself free from that and take your son to do things, and very important, talk to him, explain him all this changes even if you think he doesn't understand them, he does on his way but he does, and what a better way to build a relationship with your son than one where he can ask you?

As per your W, she might not see what she is loosing now, but thats not your problem, if she doesn't want to be a "mom" now well thats her choice, she will evenctually fail the consequences, its not your mission to show her that, all you can do its take your life in your hands and take care of yourself, the more you love yourself the more your son will recover, do you know why kids choose actors or movie roles as their image of how to live life? Basically because they are missing that figure of an emotional hero in their life's , you are your own hero, wear your speedos and your happiness mask and go have fun with son, despite what your W does wink you can do this

When the student its ready, the teacher will appear...
Even after all this time the sun never says to the Earth, "You owe me."
Look what happens with a love like that,It lights the whole sky.