What would I do?

In light of all you have written above, I would leave him but I would still keep my relationship with the step-son.

Leaving your partner doesn't mean you need to leave, or 'abandon' the child. It is a fallacy to believe that children are better off with parents who stay together, no matter what the cost.. It is better to make a clean break and leave so as to model a healthy, emotionally supportive, relationship. Staying together for the sake of staying together can cause more harm and be more detrimental to children, in many different arenas of their development.