When my ex husband and I divorced, my oldest (12 now) wanted to live with dad and my youngest (9 now) with me. My son of 12 has decided he would like to live with me and his father is determined to change his mind. He doesn't seem like he is going to fight his son too much and let him come live with me but my ex wants to keep the legalities of custody the same, which means he still has custody of him while he lives with me, I just don't see how that can work, can he do that or is that illegal, he just wants to make sure he doesn't have to pay child support, with my son going to public school in a different area and all?

Yes, he says, OK son, go live with mom, and drops him off at your house, done 1000's of time a week all across the US.

In this case, he still gets all the final say, he can demand the child move back with him at any time if he wanted,

And his dad withdraws him from that school and you enroll him at this school. You tell them you are his mom, and leave it at that, and enroll him.

You are trying to make this way too much harder than it has to be.