Here's the thing: it doesn't sound like Mikey's dad is actively abusive. There's no evidence in the letter that Mikey wouldn't be fed or clothed or loved if LW wasn't there. He would probably be *spoiled*, he might not be fed and clothed to the standard LW believes in, but that's an entirely different magnitude of problem.

I would be tempted to tell LW: you can't keep being this kid's only parent. So stop. Tell his dad that now that he's older you don't feel comfortable being the disciplinarian when you're not officially the parent, and from now on you're going to follow his lead. Help him spoil the crap out of the kid. It sounds like you all might enjoy that anyway. And I don't think Mikey will feel abandoned if he suddenly starts getting only love from you and not discipline.

Unless it really does get to the "actively abusive" stage, stick with this. Give the boyfriend advice if he asks, but don't do the work. If it does get to the "actually neglectful" stage, rather than trying to make it ok you need to make sure Mikey gets what he needs and then go to the kid's mother and tell her she needs to renegotiate the custody agreement for the kid's own safety and you'll be on her side.

And if the mom comes to you with problems she's seeing short of abuse, back her up to the BF about the changes he needs to make, but don't take responsibility for them.