You sound wonderful. You do not mention SS age. Is he in elementary, middle or high school. Some of this can make a difference.
I think most Stepmoms can step up and step in.. as long as they know when to step back.
I agree to have his father do the hard discipline.
You are allowed to compliment son just as you do your bio children, but when the few times he messes up to say, "I know you can do better."
"I have seen you behave better than this." "Tell me what is going on, you do not normally make these type of choices."
The other thing I loved about my relationship with my mom, was that we could talk about anything. Even if it was a sensitive subject.
She admitted she was not perfect. She apologized for mistakes she would sometimes make with us. She would admit when she was changing her mind and would explain it to us. I felt like I had a real mom, that was not hiding information, was not underestimating my ability to understand.
This is a chance to start explaining women and how our minds work to him.
As Women, we change over time.. And we embrace change. This is news to men, because they hang on to what works and what is comfortable.
I do not know what his moms problems are, but if it is a change in personality.. it could be depression. If she used to be responsible and now she has gone a bit wild, could be she is tired of being responsible.
You have a spouse, if she does not, it could be getting to her. If she does have a significant other, could be a relationship problem.. But whatever it is, you being able to be there for SS, without judgement.. he will appreciate you.