My step son would like to live with us, he is 15. His mother is very abusive, physically, mentallly and emotionally. What do we have to do to make this happe? Can he choose to live with us?

First, its not a good idea to piggyback your question on another thread. To avoid confusion I've moved your question to its own thread.

You can do nothing, you have no standing at this time. His father needs to submit a petition to Family court to modify custody. This petition must be submitted to the court that issued the original custody order unless both parents no longer live in that jurisdiction. Then it would go to the court where the child legally resides.

At 15, his preference will be listened to by the court and given significant weight. If the abuse can be proven (and I wonder why it has not come out before), then it shouldn't be much of a decision for the court.