So I"m 100% for him getting counseling in school. Have you asked him how he feels about it? He very well could ask for it himself through school. But either way, I would get it done. I know you're worried W is going to try and somehow "blame" you for his counseling. But the reality is his parents split, that's traumatic (believe me, I've been there!). If she tries to play the blame game, just say that it's been a hard time for everyone involved and it never hurts to have someone to talk to.
And then I think you take it one small step at a time. Ask the L about the possibility of petitioning for custody but I wouldn't pull the trigger on that. Information is power.
M:26 H:32 T:8yrs ILYBNILWY/ "I'm not happy": 2/6/16 D-Day-discovered PA/EA: 2/10/16 Separated: 2/14/16 He's living with OW I moved to different state: 06/16 Currently: Trying to move forward