Seeking expert counseling is a sign of strength. A personal relationship with a caring professional is proven clinically effective.
I have read and re-read your question. It has remained unanswered for more than 14 hours, and that is because you seem to be looking for a solution that will allow you to remain a stepmother and to retain your peace of mind, all at the same time.
Your partner is a very difficult person, very controlling. From your description he seems to be a narcissist. He wants to dominate and control you.
Narcissists are always in charge, always right, and they get what is called "narcissistic supply" from dominating and controlling others, especially their partners.
He is always right but may act as if HE were the victim and you are doing him wrong. This is the way of narcissists.
The most significant characteristic of a narcissist is their inability to understand or feel the emotional pain of others. They have no empathy and they cannot learn it. They can learn to PRETEND they are empathetic, but the cannot feel it, just as a blind man cannot see the blue sky.
They are also great liars and are successful at conning and manipulating others.
Narcissists never change, and often keep their victims imprisoned by giving them just enough to keep them hanging on.
You can continue to stay with him, but you will always feel this way. You have no legal recourse viv-a-vis your "stepson", unless your partner and the boys mom give you adoptive rights, and that probably will never happen.
Let me give you two very important books that will help you try to figure out your situation.
When you understand the nature of your situation and the difficulties that you will continue to face,l you may be able to make a more informed decision.
There is no miracle answer to this, but it depends entirely on you making the best choices.
I wish you great success and shall keep you in my prayers.
Elliott, MAE, LPCC, NCC, CCMHC