Even in the healthiest situations (and his relationship with his mom isnt), you won't supercede Mama's influence on her son. Add on a very dysfuntional mother / son dynamic and you are really speaking a different language than either of them.
He may want to stop but he hasn't taken any steps to do so. Any bit of emotional support or encouragement you give will be underminded by her. I dunno, I would step away from him right now. Just for the fact that mom is still a huge influence on his life as adult. That alone would bug me.