Jurisdiction bermuda triangle

My son who is 15 yrs old wants to live with me. His mother has full custody of him and I have visition. All sides are agreeable to my son living with me. His mother and I want the change made legal on paper in court, so that there will be no problems. The problem, the orginal order for custody is in New York, My son and his mom moved to Pennsylvania, and have been there for over 1 year making this my son new home state. I moved to florida and have been there for over 1 year. Custody was never established or moved to Pennsylvania. Pennsylvania only has a new order for child support; and that is to only move the order from ny to pa. I want to know which state has jurisdiction to change or modify the custody order. I do now that once my son is with me for six months in florida I need to establish custody in florida, but befor that how do we modify the order in New York?

If New York still has jurisdiction over the custody case, it can modify the order. But if everybody is in agreement, it may be easier to stipulate to have the custody matter transferred to Pennsylvania, and then to stipulate there to a new support and custody order consistent with your present wishes.

Is there any way to not involve the state of Pennsylvania at all?
the mother is going to be moving to florida within a year her self.

If New York still has jurisdiction over the custody case, it can modify the order. But that wouldn't affect the support order if you've actually transferred those issues to Pennsylvania.