Ive finally moved out and we will have that "talk" with our son

So I finally moved out, but we havent talked to our son yet. I just had to leave first because that feeling of constantly being erased was too much

I told my son over the weekend that I'll be staying in our other condo (a studio we are renting out) for the week cos I need to do repairs. But of course we need to finally talk to him

But when we finally talk to him (he is 15yrs old) together, I know the main thing will be (and we both agree on this)

1. That its not his fault, and he isnt losing his parents (ill be living 15 mins away anyway) and th only change is that I wont be living a the main condo but I will still be very much present in his life

2. Morning routine - wife will still drive him to school ; after school routine - he will spend time with me in the cafe and Ill drive him back home (only difference this time is that I wont be staying at the house and Ill leave after dropping him)

But come to the topic of

1. Why this had to happen?

How do I answer that when I didnt want this to happen, in fact, I endured the constant erasure,and my wife's affair for a year?

2. I want to spare him the details of knowing of my wife's affair but how do I tell him that " I walked away from the marriage because your mother dont want to repair anymore and that I cant shake off that anguish from he affair" without making his mother look bad?

Context of my story here:

https://www.reddit.com/r/kwarentahin/comments/1t3cjw4/starting_all_over_again_mid_40s/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button