Moderator, please edit the original post to remove suburbs.
At 16 years of age, a court will consider your son old enough and mature enough to basically decide his living arrangements for himself. His opinion will be heard by the judge and given significant weight when the judge makes his decision.
But hopefully it doesn't come to a court hearing.
I suggest calling the mother to mediation to discuss your son's living arrangements. It may even be worthwhile having a child inclusive conference so your son can speak for himself. Some things to consider are how new care arrangements will work, how often your son will see his mother, what school he will go to, how travel will work, what boundaries the mother has enforced and how those boundaries will continue being enforced if he lives with you (after all, there is a chance your son just doesn't like that his step-dad says he has to be home by 11pm on school nights or some such).
Hope this helps.