At thirteen your S can be told the facts. Don't embellish things and make her worse than she is but don't hide any facts. It's worse for you to "cover" for her, then later for your S to find out the truth from others. Better to explain things factually, the truth will come out eventually.

I did that with my S11 and it made him sad but he was fine. I think in the long run it will be better for him to know the truth.

Me-LBH, 48
Spouse-WW, 48
Married for 19 years
Son, 12
BD #1 - November 1998 (EA 7 months after wedding)
BD #2 - November 2015 (same XBF EA)
WW filed D February 2016
WW moved out April 2016