Hi Tutak,
I'm so sorry to hear what your son - and you of course - is going through.
I was in this position myself as a child.
I think that by far the best thing you can for your son is to tell him the truth, geared towards his age of course. Yes, it will hurt him. But that is not your fault, and you are there to love and support him.
There is a great strength that comes from knowing the truth, even when that truth makes you feel sad.
Sit down with him in a supportive, loving way and tell him the truth. You know him best so you know what he will understand.
I don't think it is a bad thing to let a child know that you can't know what is going on inside someone else's head. You can't explain how your ex's mind works.
Your son may need some reassurance that he is not bad or wrong to feel sad about this. I think it can help a child to be told that yes, what has happened is not fair. But that you will always love him - lots of honesty but also lots of reassurance. I think kids know when they are being fobbed off and it leads to confusion and uncertainty.
Make it clear to him that you are always willing to talk about it at any time. That you expect questions will occur to him and that he should just feel able to ask you them whenever he wants to.
But tell him the truth in a form he can get his head around. This is his dad's decision and you don't understand it - you can't explain it. But you and your son make up a team.
One of the things that made me feel strongest as a child was that I had an open and honest relationship with my mum - that she didn't conceal the more difficult things from me, she just helped me to find a way to live with them as part of my life.
I hope some of this helps. There are a couple of other threads that might contain some helpful info - I will find the links.