It's very hard, I know.
Now that I remember, my daughter was 16 when her dad and I separated, and it was hard for her to verbalize what she wanted.
I suggested she have the opportunity to speak with a therapist about it all, and she liked that idea. It helped her to be able to say whatever she wanted to about both of us in confidence, and to be validated. Took a lot of pressure off me as well, trying to get it all right.
I wish you the best. Your son is fortunate to have a mom that is so caring of his feelings.