"S16 told me for the first time in his life he felt like his mother hated him with the way she treated him yesterday. BUT.....when I asked S16 how he would feel about me discussing it with W he told me he didn't want me to"
There are some things that you can/should let your kids work out and then there is this type of issue. You can't let your son believe this of his mother!!! Even though he asked you not to..I feel this has to be addressed.
I would go to the son and say...look I know you ask me not to talk to your mother about this but I can't let this linger between you two. Either you talk to her or let me do it.