As someone who was forced at that age to do counseling that I didn't want to do, if you force it, it'll be a negative memory that could hurt more than help. I'm not saying give up, but you have to give him some time to warm up to the idea. Keep just subtly bringing it up. If you talk to other S and he agrees to do counseling, that may help.
But if you force the subject, it'll backfire. The best thing you can do is just keep talking to him. And keep telling him you love him and that you'll both get through this. And as hard as it is, try to remind him that his mom is still his mom and she loves him even if she's no good at showing it right now. Explain that she's hurting and doesn't know how to handle it, so she's lashing out.
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