I think that's exactly the case. If I'm Tunsil, I fight that weasel every step of the way. I doubt the stepdad has the resources to carry this very far. No matter what, it's a difficult position for Tunsil if his his Mother is still married to/living with the guy.
True, but he's a man. And a stepfather has to understand that - unless he is a super duper dad and/or Tunsil's biological father was either absent or abusive - a child is always going to favor his biological father.
Too many stepdads try to sweep in and try to force being "real" dad. Understand your role as a stepfather and let the child decide if he sees you as his/her "real" dad.
Just by reading the story, it sounds like the stepdad is trying to make decisions on the behalf of Tunsil instead of giving sound advice. And when his advice isn't received, he's getting mad. Maybe some agent tried to reach the stepdad and ingratiate himself into the picture and Tunsil wasn't having it. That happens too, unfortunately. Work on the parent to get in with the child.